Index merkle tree insertions just like messages

[tkporter]

Description

Moves merkle tree hook insertion indexing away from the watermarked contract sync to the same method that message indexing uses

• Generalizes all the MessageContractSync / MessageSyncCursor etc to work with any "sequenced" data. Sequenced data means a data type that has a sequence # that increases 1 at a time as it's created. E.g. a message's nonce is its sequence, or a merkle tree insertion's leaf index is its sequence

Drive-by changes

Minor idiomatic changes

Related issues

n/a

Backward compatibility

I believe this is fully backward compatible

Testing

Tested locally by running a relayer with an empty data dir & with an existing data dir. Also tested a validator locally