[STORE] Allow to get metadata from arbitrary commit points

Today we always use the latest commit point to return the metadata from
the store. This might cause problems for snapshot and restore since in
contrast to recovery it won't prevent concurrent flushes (lucene commits).
This can lead to all kinds of interesting effects if we are snapshotting
while flushing. This change uses the IndexCommit to open the metadata snapshot
from the store which is consistent with what we snapshot.