os/bluestore: shard extent map

os/bluestore: shard extent map

Rewrote much of the persistence of onode metadata.  The
highlights:

 - extents and blobs stored together (the blob with the
   first referencing extent).
 - extents sharded across multiple k/v keys
 - if a blob if referenced from multiple blobs, it's
   stored in the onode key (called a "spanning blob").
 - when we clone a blob we copy the metadata, but mark
   it shared and put (just) the ref_map on the underlying
   blocks in a shared_blob key.  at this point we also
   assign a globally unique id (sbid = shared blob id)
   so the key has a unique name.
 - we instantiate a SharedBlob in memory regardless of
   whether we need to load the ref_map (which is only
   needed for deallocations!).  the BufferSpace is
   attached to this SharedBlob so we get unified caching
   across clones.

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