It seems p2p-compactblocks.py fails randomly

From: https://travis-ci.org/bitcoin/bitcoin/jobs/163788152 from #8841
The PR is trivial and totally unrelated to compact block

```
p2p-compactblocks.py:
Initializing test directory /tmp/test3yd_2q3h/2
start_node: bitcoind started, waiting for RPC to come up
start_node: RPC succesfully started
MiniNode: Connecting to Bitcoin Node IP # 127.0.0.1:11016
Testing SENDCMPCT p2p message... 
Testing compactblock headers and shortIDs are correct...
Assertion failed: 
Stopping nodes
Not cleaning up dir /tmp/test3yd_2q3h/2
Failed
stderr:
  File "/home/travis/build/bitcoin/bitcoin/qa/rpc-tests/test_framework/test_framework.py", line 151, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/../qa/rpc-tests/p2p-compactblocks.py", line 614, in run_test
    self.test_compactblock_construction()
  File "/home/travis/build/bitcoin/bitcoin/build/../qa/rpc-tests/p2p-compactblocks.py", line 264, in test_compactblock_construction
    assert(self.test_node.last_cmpctblock is not None)
Pass: False, Duration: 7 s
```

switch (inv.type)
    {
    case MSG_TX:
    case MSG_WITNESS_TX:
        {
            assert(recentRejects);
            if (chainActive.Tip()->GetBlockHash() != hashRecentRejectsChainTip)
            {
                
                
                
                
                hashRecentRejectsChainTip = chainActive.Tip()->GetBlockHash();
                recentRejects->reset();
            }
            
            
            return recentRejects->contains(inv.hash) ||
                   mempool.exists(inv.hash) ||
                   mapOrphanTransactions.count(inv.hash) ||
                   pcoinsTip->HaveCoinsInCache(inv.hash);
        }
    case MSG_BLOCK:
    case MSG_WITNESS_BLOCK:
        return mapBlockIndex.count(inv.hash);
    }
    
    return true;
}
static void RelayTransaction(const CTransaction& tx, CConnman& connman)