rewrite: optimize the interval of sync when rewriting memtables. (t…

…ikv#347)

In a cloud environment, refraining from unscheduling sync operations
when rewriting memtables might result in an accumulation of unsynced bytes
in the buffer. This accumulation has the potential to impede the foreground
write progress during sync.

This pull request introduces periodic sync operations when the amount of
stashed unsynced bytes exceeds a predefined threshold. This optimization
aims to address the issue and enhance performance.

Signed-off-by: lucasliang <[email protected]>