Problem: Metadata are requested more than once for each blob

Setting a breakpoint in AbstractSharedCachingDataStore.getRecordIfStored() and logging the dataIdentifiers, we see that it calls backend.getRecord() 3 times for the same dataIdentifier when a replication package is installed by vault. The reason seems to be that during commits, every CommitHook runs its own compareAgainstBaseState and, because the implementation avoids fetching the blob if it only needs the metadata, the request to the existing blob cache is always a miss.

Proposed solution: Cache backend.getRecord() calls

Manual testing has shown that caching backend.getRecord() calls reduces the time spent in .getRecordIfStored() by between 12 and 35% when installing replication packages containing 500 paths.

The PR is at https://github.com/apache/jackrabbit-oak/pull/1155