os/bluestore: allocate entire write in one go

os/bluestore: allocate entire write in one go

On the first pass through the writes, compress data and calculate a final
amount of space we need to allocate.  On the second pass, assign the
extents to blobs and queue the writes.

This allows us to do a single allocation for all blobs, which will lead
to less fragmentation and a much better write pattern.

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