crimson/osd: make the ObjectContextRegistry per-PG

crimson/osd: make the ObjectContextRegistry per-PG

This patch moves the OBC registry from ShardServices (which
is basicaly a gateway to a bunch of PGs) into PG itself.
Dividing OBCs by PG (they truly belong to) minimizes the space
to search when e.g. checking whether a client is blocked or
not. Therefore, this commit is enabler of more performant
PR #47637.

The changeset draws the assumption that OBC registry and all
its users live on the same CPU core. It looks valid to me.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>