osd/: move split_list and split_request into OSD

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

This way, we can handle pg splits atomically with ms_fast_dispatch.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1944 from athanatos/wip-8396

Move the waiting_for_pg list down into the Sessions, and coordinate the updates so that we don't get races
between the Pipe and handle_map threads.

Fixes: #8396
Fixes: #8505

Reviewed-by: Greg Farnum <greg@inktank.com>