OSD: fix wake_pg_waiters revert error in _open_lock_pg

231fe1b685bfbd3db9c81709ca39a29d696b13ad reintroduced erroneously
this call to wake_pg_waiters.  All _create_lock_pg callers handle
calling wake_pg_waiters after the pg lock has been dropped.

Fixes: #8691
Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: wake_pg_waiters outside of the pgmap write_lock, pg_lock

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: rename session_waiting_for_map_lock to session_waiting_lock

This lock will also protect the waiting_for_pg structures in each
session.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: clear_session_waiting_on_map in ms_handle_reset

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD::shutdown: actually drop sessions waiting on map

There might be messages for which we still don't have the
map.  dispatching waiting won't actually help.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: move waiting_for_pg into the session structures

Each message belongs to a session.  Further, no ordering is implied
between messages which arrived on different sessions.  Breaking the
global waiting_for_pg structure into a per-session structure lets
us avoid the problem of taking a write lock on a global structure
(pg_map_lock) in get_pg_or_queue_for_pg at the cost of some
complexity in updating each session's waiting_for_pg structure when
we receive a new map (due to pg splits) or when we locally create
a pg.

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