WAL replaying optimization for incomplete transactions

[royi-luo]

Description

Our WAL replaying can be quite slow. This can lead to cases like in #5016 where upon killing a long-lasting query and then reloading the DB it can appear like the DB is hanging (when in reality we are just replaying the query).

In the above case uncommitted records actually don't need to be replayed. Thus one possible optimization would be to deserialize records in batches and only replay the records if they are followed by a commit. In the above case this would mean that we only pay the cost of deserializing records and not the cost of replaying them.