Table.Upsert based on pebble.Merge

[marino39]

The idea is for Table.Upsert to use pebble.Merge function. This function writes to SST entry marked as a merge. The entry is merged with old version of the key on key get, when key is encountered in the iteration and during compactions.

This way we could limit amount of reads during indexing TokenBalances in Sequence Indexer to 0. The keys would be merged as they are read or compacted.

Estimated impact:

• ~50% gain in speed of indexing
• Unknown probably minor loses in read speed as now keys will need to be merged on read. However, the more frequently address is written to probably it will be read more frequently too. So I believe impact on read speed should spread and be really low.

Merger interfaces:
https://github.com/cockroachdb/pebble/blob/1798fbf5956c16664b6707c51947b65c9e1a10d4/internal/base/merger.go#L88