os/bluestore: Add admin socket commands to inspect onode metadata

os/bluestore: Add admin socket commands to inspect onode metadata

Add admin socket commands:
1) bluestore collections
Lists collections.
2) bluestore list <coll> [start object] [max count]
Lists collection coll starting from object (optional). Default 100 entries. 0 = unlimited.
3) bluestore onode metadata <object>
Prints onode metadata as seen by BlueStore.

It might happen (usually in tests) that 2 BlueStore instances are created at the same time.
Since admin commands are unique, it fails to register.
Use first register to detect whether we can register at all.

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