crimson/os/seastore/onode: measure onode-tree lookup cost

crimson/os/seastore/onode: measure onode-tree lookup cost

Before optimizing the onode lookup paths we should measure the existing
cost if each possible optimization.

The per-node search is already binary search(see search_result_bs_t
binary_search). Possible costs are:
- per-level node traversal
- STAGE_STRING ns/oid memcmp (rbd has long shared oid prefixes)

Adding the following stats to be exposed as seastar-metrics (not logs).

Keep the new sampling per-comparison overhead out unless needed for
measurment runs.

Candidate optimization leads from the numbers:
* cross-node string/prefix dedup (marked as TODOs in stage.h) this could
  result in cheaper compares for shared (rbd) prefixes.
* shrink onode_layout_t - marked as onode.h TODO.
  stop inlining oi and ss to get more objects per leaf.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>