feat: impl checkTxAsyncReactor()

[jinsan-line]

Closes: #167
Related with: #163, https://github.com/line/link/issues/1152

Description

During #163 CI tests, we sometimes failed TestReactorBroadcastTxMessage(). After investigation, I think
we should guarantee that tx, called mempool.CheckTxAsync() first, should call app.CheckTxAsync() first. I think this bug comes from original tendermint. But, in original tendermint, app.CheckTxAsync() is protected by a mutex. Because part of mempool.CheckTxAsync() is serialized by this mutex, it decreases the possibility of failure of the test. Because we removed this mutex, the possibility of failure of the test is increased.

To guarantee the execution order, I introduce a channel and a reactor. It makes the mempool.CheckTxAsync() execute in serial. mempool/reactor.Receive() is executed on its own goroutine. In original tendermint, mempool.CheckTxAsync() could be executed in parallel except app.CheckTx(). Now, mempool.CheckTxAsync() is executed in serial. It looks like decreasing the performance.

Please note that mempool.CheckTxAsync() itself is so cheap but app.CheckTxXXX() is expensive. Now we compare the original and revised. In the original, cheap mempool.CheckTxAsync() itself could be executed in parallel but expensive app.CheckTx() is executed in serial. But, now, cheap mempool.CheckTxAsync() itself is executed in serial but expensive app.CheckTx() could be executed in parallel. So I maintain that the revised is safer and more performant than the original.

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For contributor use:

• Wrote tests
• Updated CHANGELOG_PENDING.md
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments
• Re-reviewed Files changed in the Github PR explorer

[egonspace]

Is prepareCb right? I think it's just an error handler.

[jinsan-line]

I think it's prepareCheckTx callback. If we follow up the argument for callback name, checkTxCb should ABCIResponse callback.

[kukugi]

LGTM 👍

[wetcod]

LGTM