When the mempool fills the verbose `getrawmempool` RPC call gets really slow.  It can take minutes for >100k transactions in the mempool.

The culprit is the univalue library.  Objects are implemented as a big list, but the library checks for duplicate keys.  So inserting n key-value pairs into an object takes O(n^2).

Possible fixes:

- Improve univalue library by using linked hash maps.
- change the RPC output format of getrawmempool to use a list of objects, instead of an object of objects.  But this breaks existing code.
- hack the library to be able to skip the duplicate check.

"    \"depends\" : [           (array) unconfirmed transactions used as inputs for this transaction\n"
           "        \"transactionid\",    (string) parent transaction id\n"
           "       ... ]\n"
           "    \"spentby\" : [           (array) unconfirmed transactions spending outputs from this transaction\n"
           "        \"transactionid\",    (string) child transaction id\n"
           "       ... ]\n"
           "    \"bip125-replaceable\" : true|false,  (boolean) Whether this transaction could be replaced due to BIP125 (replace-by-fee)\n";
}
static void entryToJSON(const CTxMemPool& pool, UniValue& info, const CTxMemPoolEntry& e) EXCLUSIVE_LOCKS_REQUIRED(pool.cs)
{
    AssertLockHeld(pool.cs);
    UniValue fees(UniValue::VOBJ);
    fees.pushKV("base", ValueFromAmount(e.GetFee()));
    fees.pushKV("modified", ValueFromAmount(e.GetModifiedFee()));
    fees.pushKV("ancestor", ValueFromAmount(e.GetModFeesWithAncestors()));
    fees.pushKV("descendant", ValueFromAmount(e.GetModFeesWithDescendants()));
    info.pushKV("fees", fees);
    info.pushKV("size", (int)e.GetTxSize());
    info.pushKV("fee", ValueFromAmount(e.GetFee()));
    info.pushKV("modifiedfee", ValueFromAmount(e.GetModifiedFee()));
    info.pushKV("time", e.GetTime());
    info.pushKV("height", (int)e.GetHeight());
    info.pushKV("descendantcount", e.GetCountWithDescendants());
    info.pushKV("descendantsize", e.GetSizeWithDescendants());
    info.pushKV("descendantfees", e.GetModFeesWithDescendants());
    info.pushKV("ancestorcount", e.GetCountWithAncestors());
    info.pushKV("ancestorsize", e.GetSizeWithAncestors());
    info.pushKV("ancestorfees", e.GetModFeesWithAncestors());
    info.pushKV("wtxid", pool.vTxHashes[e.vTxHashesIdx].first.ToString());
    const CTransaction& tx = e.GetTx();