core/rawdb: introduce flush offset in freezer

[rjl493456442]

It's a following PR of #29792 to get rid of the data file sync.

This is a non-backward compatible change, which increments the database version from 8 to 9.

This pull request introduces a flushOffset for each freezer table, which tracks the position of the most recently fsync’d item in the index file.

When this offset moves forward, it indicates that all index entries below it, along with their corresponding data items, have been properly persisted to disk. The offset can also be moved backward when truncating from either the head or tail of the file.

Previously, the data file required an explicit fsync after every mutation, which was highly inefficient. With the introduction of the flush offset, the synchronization strategy becomes more flexible, allowing the freezer to sync every 30 seconds instead.

The data items above the flush offset are regarded volatile and callers must to ensure they are recoverable after the unclean shutdown, or explicitly sync the freezer before any proceeding operations.

[fjl]

This needs to be deployed on benchmark machine another time.