Summary:
It could happen that Vector LSM was not flushed before TServer restart/crash.
In this case during bootstrap we should add understand what operations should be reapplied to it.
To address this issue we have UserFrontiers in rocksdb.
Could use the same approach for Vector LSM.

Also added ability to flush Vector LSM when necessary.

One important TODO added:
It could happen that older chunk not yet saved. So we should delay adding this file to metadata, until older chunk is saved.
Jira: DB-13376

Test Plan: *VectorLSMTest.Bootstrap*

Reviewers: mbautin, aleksandr.ponomarenko

Reviewed By: mbautin

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39065