Problem: Missing "multi-send" transfer event support

## Background

In Cosmos SDK, there is a "multi-send" (`InputOutputCoins`) support in bank keeper (Ref: https://github.com/cosmos/cosmos-sdk/blob/1321bc1614df068f1bce50170f891a77b1e831a3/x/bank/keeper/send.go#L70). In the current `begin_block` and `end_block` parser it does not support it yet.

In particular, the "multi-send" is characterised by
- No `sender` key in the `transfer event`
- Followed the pattern:
- `message` event
- followed by multiple pairs of
- `coin_received` event (post-v0.43)
- `transfer` event