the `zapwallettxes.py` qa test is currently broken due to a syntax error:

```python
     assert_raises(JSONRPCException, self.nodes[0].gettransaction, [txid3])
     #there must be a expection because the unconfirmed wallettx0 must be gone by now
```

This doesn't actually test that the transaction is not in the wallet, because `gettransaction()` is being called incorrectly (with an array instead of a string).

Changing the test to `assert_raises(JSONRPCException, self.nodes[0].gettransaction, txid3)` causes the rpc to succeed and the test script to fail because `txid3` is still in the wallet.

This is because the mempool is now persistent and is saved to disk when bitcoind shuts down. zapwallettxes removes the unconfirmed transaction from the wallet but bitcoind will rescan the mempool.dat file on startup and add the unconfirmed transaction back into the wallet.

I've 'fixed' zapwallettxes.py in https://github.com/jnewbery/bitcoin/tree/zapwallettxes , but the question remains: how *should* -zapwallettxes interact with persistent mempool? The use case for zapwallettxes is explained here: https://github.com/bitcoin/bitcoin/pull/3659#issuecomment-34878999 . My view is that even with zapwallettxes, bitcoind should rescan the mempool, but there should be better documentation or a warning that unconfirmed transaction in your mempool will be re-added to the wallet even if zapwallettxes is used. If you really want to remove all unconfirmed transactions from your wallet, you can delete your mempool.dat file and then restart with zapwallettxes.
strUsage += HelpMessageOpt("-persistmempool", strprintf(_("Whether to save the mempool on shutdown and load on restart (default: %u)"), DEFAULT_PERSIST_MEMPOOL));
    strUsage += HelpMessageOpt("-blockreconstructionextratxn=<n>", strprintf(_("Extra transactions to keep in memory for compact block reconstructions (default: %u)"), DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN));
    strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
        -GetNumCores(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
#ifndef WIN32
    strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file (default: %s)"), BITCOIN_PID_FILENAME));
#endif
    strUsage += HelpMessageOpt("-prune=<n>", strprintf(_("Reduce storage requirements by enabling pruning (deleting) of old blocks. This allows the pruneblockchain RPC to be called to delete specific blocks, and enables automatic pruning of old blocks if a target size in MiB is provided. This mode is incompatible with -txindex and -rescan. "
            "Warning: Reverting this setting requires re-downloading the entire blockchain. "
            "(default: 0 = disable pruning blocks, 1 = allow manual pruning via RPC, >%u = automatically prune block files to stay under the specified target size in MiB)"), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
    strUsage += HelpMessageOpt("-reindex-chainstate", _("Rebuild chain state from the currently indexed blocks"));
    strUsage += HelpMessageOpt("-reindex", _("Rebuild chain state and block index from the blk*.dat files on disk"));
#ifndef WIN32
    strUsage += HelpMessageOpt("-sysperms", _("Create new files with system default permissions, instead of umask 077 (only effective with disabled wallet functionality)"));
#endif
    strUsage += HelpMessageOpt("-txindex", strprintf(_("Maintain a full transaction index, used by the getrawtransaction rpc call (default: %u)"), DEFAULT_TXINDEX));
    strUsage += HelpMessageGroup(_("Connection options:"));
    strUsage += HelpMessageOpt("-addnode=<ip>", _("Add a node to connect to and attempt to keep the connection open"));
    strUsage += HelpMessageOpt("-banscore=<n>", strprintf(_("Threshold for disconnecting misbehaving peers (default: %u)"), DEFAULT_BANSCORE_THRESHOLD));
    strUsage += HelpMessageOpt("-bantime=<n>", strprintf(_("Number of seconds to keep misbehaving peers from reconnecting (default: %u)"), DEFAULT_MISBEHAVING_BANTIME));
    strUsage += HelpMessageOpt("-bind=<addr>", _("Bind to given address and always listen on it. Use [host]:port notation for IPv6"));
    strUsage += HelpMessageOpt("-connect=<ip>", _("Connect only to the specified node(s); -noconnect or -connect=0 alone to disable automatic connections"));
    strUsage += HelpMessageOpt("-discover", _("Discover own IP addresses (default: 1 when listening and no -externalip or -proxy)"));
    strUsage += HelpMessageOpt("-dns", _("Allow DNS lookups for -addnode, -seednode and -connect") + " " + strprintf(_("(default: %u)"), DEFAULT_NAME_LOOKUP));
    strUsage += HelpMessageOpt("-dnsseed", _("Query for peer addresses via DNS lookup, if low on addresses (default: 1 unless -connect/-noconnect)"));
    strUsage += HelpMessageOpt("-externalip=<ip>", _("Specify your own public address"));
    strUsage += HelpMessageOpt("-forcednsseed", strprintf(_("Always query for peer addresses via DNS lookup (default: %u)"), DEFAULT_FORCEDNSSEED));
    strUsage += HelpMessageOpt("-listen", _("Accept connections from outside (default: 1 if no -proxy or -connect/-noconnect)"));
    strUsage += HelpMessageOpt("-listenonion", strprintf(_("Automatically create Tor hidden service (default: %d)"), DEFAULT_LISTEN_ONION));
    strUsage += HelpMessageOpt("-maxconnections=<n>", strprintf(_("Maintain at most <n> connections to peers (default: %u)"), DEFAULT_MAX_PEER_CONNECTIONS));