Add more logging for testSnapshotAndRestore backward compatibility test
[STORE] Improve recovery / snapshot restoring file identity handling

This commit changes the way how files are selected for retransmission
on recovery / restore. Today this happens on a per-file basis where the
rather weak checksum and the file length in bytes is compared to check if
a file is identical. This is prone to fail in the case of a checksum collision
which can happen under certain circumstances.
The changes in this commit move the identity comparsion to a per-commit / per-segment
level where files are only treated as identical iff all the other files in the
commit / segment are the same. This "all or nothing" strategy is reducing the chance for
a collision dramatically since we also use a strong hash to identify commits / segments
based on the content of the ".si" / "segments.N" file.

Closes #7351
[SNAPSHOT] Add BWC layer to .si / segments_N hashing

Due to additional safety added in #7351 we compute now a strong hash for
.si and segments_N files which are compared during snapshot / restore.
Old snapshots don't have this hash which can cause unnecessary copying
of large amount of data. This commit adds the ability to fetch this
hash from the blob store if needed.

Closes #7434
[SNAPSHOT] Ensure BWC layer can read chunked blobs
Use empty BytesRef if we read from <= 1.3.3
[TEST] only bump replicas if we have enough nodes in the cluster
[TEST] only expand to 1 replica in SnapshotBackwardsCompatibilityTest
Use physical name to compare files from snapshot metadata

The comparison and read code in the BlobStoreIndexShardRepository
used the physicalName and Name in reverse order. This caused
SnapshotBackwardsCompatibilityTest to fail.
Adjust inline comments

We backported this feature to 1.3.3 as well so comments should be
updated.