mempool: handle out-of-order ABCI responses

[erikgrinaker]

The mempool currently seems to require asynchronous ABCI calls to returns responses in the same order they were called - e.g. CListMempool.recheckCursor has an ordered list of transactions to recheck, but calls CheckTxAsync to actually recheck them, which may return responses in a different order.

This caused #5439, since the gRPC ABCI client, although serial in nature, used unordered goroutines to simulate async responses in order to satisfy the async API semantics.

[alexanderbez]

When I spoke to @ebuchman about this for clarification on the v1 mempool, I recall that we're guaranteed to get the responses in the order in which they were executed.

[tac0turtle]

for unix, tcp and in process this is the case, but with gRPC the issue above arises.