buffer: adjust #include order

The pthread.h include is somehow clobbering things, although it is not
clear how.  :(

Signed-off-by: Sage Weil <sage@inktank.com>
ReplicatedPG::agent_load_hit_sets: take ondisk_read_lock

Otherwise, the hit_set might be not yet written due to a recently
completed recovery.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-sam-testing2', 'origin/wip-7942', 'origin/wip-8124', 'upstream/wip-7891', 'origin/wip-7588', 'origin/wip-8011' and 'origin/wip-7439' into wip-sam-testing-safe