os/bluestore: make tail cache a bit smarter

os/bluestore: make tail cache a bit smarter

This is really a stop-gap.  Since we are doing reads in the pre-commit
write path, we need to have some sort of buffer cache so that a sequence
of writes in the same transaction can remain coherent (the second write
must "read" the first write in order to fill out the chunk).

Signed-off-by: Sage Weil <sage@redhat.com>