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>