mempool: tx could be sent back to sender

[hvanz]

On ADR-105, it was decided to move the list of senders from the mempoolTx entries in CList to the mempool reactor. This was implemented in #1010. The interface of CheckTx was changed from

CheckTx(tx types.Tx, cb func(*abci.ResponseCheckTx), txInfo TxInfo) error

to

CheckTx(tx types.Tx) (abcicli.ReqRes, error)

where TxInfo used to contain the sender information. With this modification, the reactor now calls CheckTx and record the sender like this:

reqRes, err := memR.mempool.CheckTx(tx)
...
reqRes.SetCallback(func(res *abci.Response) {
	if res.GetCheckTx().Code == abci.CodeTypeOK {
        	memR.addSender(tx.Key(), sender.ID())
        }
})

The problem with this design is that the callback may introduce a delay when recording the sender. broadcastTxRoutine reads the transactions as soon as they are added to clist, so when it tries to read the list of senders in the reactor, the sender is not there yet, so it sends back the transaction to the sender.

A potential solution would be to move back the sender to the CList entries. Then the sender would be stored together with the transaction in mempoolTx and would be available at the moment the transaction is broadcasted. The drawback of this solution is that it reintroduces the mixing of the pool implementation and the gossip protocol.

Also note that:

• Removing the cb parameter from the original CheckTx signature does not pose a problem and it's still useful to keep it out of CheckTx. This callback was only used by the RPC endpoints broadcast_sync and broadcast_commit to handle the CheckTx response. Now this is done with abcicli.ReqRes.
• ADR-105 was not backported to v0.38.x.