Inconsistent transaction hashing with Tendermint

[fedekunze]

The hash that a user or library would expect from sending an ethereum tx over JSON-RPC doesn't match the one provided by the ResponseDeliverTx on Tendermint.

ref: tendermint/tendermint#6539

[yihuang]

If don't modify tendermint itself, we can store the hash relationship on chain?

[fedekunze]

that's an option, but we would have to prune the entries at some point. The map would have to be eth hash --> tm hash (querying by tm hash should already work fine)

[yihuang]

another thing about tx hash is, if we maintain hash relationship by ourself, we might be able to support multiple eth tx in one tm tx, just maintain the map of eth hash -> (tm hash, msg index), if we customize the tm hash, we stick with 1-to-1 map of evm tx and tm tx. There might be other tricky issues with multiple eth tx in single tm tx though.

[fedekunze]

Let's implement the map as a short-term solution. The update to Tendermint probably won't be included in a release soon...
cc: @marbar3778

[yihuang]

We already emit the eth tx hash in events, so as long as enabled indexing in tendermint, we can query /tx_search to do the hash conversion.
Sounds good as a temporary solution before we finally customize tendermint.
Leave the pruning issues to tendermint itself.