As per #2127, this refactors the RequestCheckTx ProtoBuf struct to allow for a flag indicating whether a query is a recheck or not (and allows for possible future, more nuanced states).

In order to pass this extended information through to the ABCI app, the proxy.AppConnMempool (and, for consistency, the proxy.AppConnConsensus) interface seems to need to be refactored along with abcicli.Client.

And, as per this comment, I've made the following modification to the protobuf definition for the RequestCheckTx structure:

enum CheckTxType {
  New = 0;
  Recheck = 1;
}

message RequestCheckTx {
  bytes tx = 1;
  CheckTxType type = 2;
}

• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG_PENDING.md