PITR: Integration with tablet splitting

[bmatican]

Rolling back to a point before a number of splits has a couple of implications we'll need to consider

• extend lifetime of parent tablets
• change tablet metadata state (be able to go from split completed back to running)
• figure out tablet partition version, as MetaCache might not be ok with the version going back in time

Also, a bigger and more operational issue, is that keeping parent tablets around leads to space amplification! For every parent tablet, we double the space usage between it and children, as soon as the children finish the post-split compaction.

One option here could be to disable splitting, if space usage gets past some % of total disk capacity. This will be much easier after #8234

cc @ttyusupov @mbautin

[bmatican]

Fixed as part of 6be74dc