Eliminate some allocations/copies around the blob cache

[ltamasi]

Summary:
Historically, BlobFileReader has returned the blob(s) read from the file
in the PinnableSlice provided by the client. This interface was
preserved when caching was implemented for blobs, which meant that
the blob data was copied multiple times when caching was in use: first,
into the client-provided PinnableSlice (by BlobFileReader::SaveValue),
and then, into the object stored in the cache (by BlobSource::PutBlobIntoCache).
The patch eliminates these copies and the related allocations by changing
BlobFileReader so it returns its results in the form of heap-allocated BlobContents
objects that can be directly inserted into the cache. The allocations backing
these BlobContents objects are made using the blob cache's allocator if the
blobs are to be inserted into the cache (i.e. if a cache is configured and the
fill_cache read option is set). Note: this PR focuses on the common case when
blobs are compressed; some further small optimizations are possible for uncompressed
blobs.

Test Plan:
make check

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[riversand963]

LGTM.

[ltamasi]

Thanks a lot for the review @riversand963 !