TryCatchUpWithPrimary: skip MANIFEST tail reads when file size did not change

[dungeon-master-666]

I noticed that TryCatchUpWithPrimary takes a lot of time in my setup even when there were no changes in DB. In my case for 500 GB RocksDB instance it takes up to ~50 ms and most of this time is tail reading of MANIFEST file.

In this PR I implement the following optimization - to cache MANIFEST file size and if it did not grow since last call immediately return control. It makes sense for 2 reasons:

• Manifest does not change often
• Syscall to get file size is much cheaper operation than reading file from specific offset

Implementation details:

• add a helper for ReactiveVersionSet to query the current MANIFEST size via the filesystem
• cache the last known size using the existing VersionSet::manifest_file_size_ and skip ManifestTailer::Iterate() when the file hasn’t grown
• reset the cached size whenever we reopen/switch the MANIFEST so the next pass reprocesses from the beginning

This optimization speeds up TryCatchUpWithPrimary up to 3 times in my tests.