os/bluestore: Add can_merge_blob and merge_blob

os/bluestore: Add can_merge_blob and merge_blob

Two new functions:
1) can_marge_blob checks if 2 blobs are compatible to be merged together
2) merge_blob merges 2 blobs into 1, emptying source and putting all to destination

Modify merge_blob() to return logical length of produced blob.

Clear "unused" bitmap

When make blob shared or merge it with other blob, clear unused.
It drops some potential optimizations for writing into large blobs,
but it is unlikely to be useful.
Such info can be useful only after it reverts from Shared to regular blob.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>