os/bluestore: switch to intrusive_set for blobs

os/bluestore: switch to intrusive_set for blobs

Switch from a map of bare bluestore_blob_t's to an intrusive_set of
Blob structs that includes id, bluestore_blob_t, and BufferSpace.  The
buffer cache portion isn't used yet, but it is properly initialized with
the Cache parent.

There is no real functional change here except that during clone we move
the existing Blob from the Onode's BlobMap to the Bnode's without any
copying.

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