OSD::send_incremental_map: use service superblock so we can avoid locking osd_lock

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: add session waiting_for_map mechanisms

This will replace the existing waiting_for_osdmap mechanism
with a per-session wait list.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: remove wake_all_pg_waiters

We shouldn't need this -- we check the pg waiters list on each
map.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: wake_pg_waiters atomically with pg_map update

Also, call enqueue_op directly rather than going back
through the entire dispatch machinery.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: add dispatch_op_fast and supporting mechanisms

This patch adds a fast path for osd ops which bypasses
the osd_lock and should not block on any longly held
locks.

Signed-off-by: Samuel Just <sam.just@inktank.com>