====================
Backfill Reservation
====================

When a new osd joins a cluster, all pgs containing it must eventually backfill
to it.  If all of these backfills happen simultaneously, it would put excessive
load on the osd.  osd_num_concurrent_backfills limits the number of outgoing or
incoming backfills on a single node.

Each OSDService now has two AsyncReserver instances: one for backfills going
from the osd (local_reserver) and one for backfills going to the osd
(remote_reserver).  An AsyncReserver (common/AsyncReserver.h) manages a queue
of waiting items and a set of current reservation holders.  When a slot frees
up, the AsyncReserver queues the Context* associated with the next item in the
finisher provided to the constructor.

For a primary to initiate a backfill, it must first obtain a reservation from
its own local_reserver.  Then, it must obtain a reservation from the backfill
target's remote_reserver via a MBackfillReserve message. This process is
managed by substates of Active and ReplicaActive (see the substates of Active
in PG.h).  The reservations are dropped either on the Backfilled event, which
is sent on the primary before calling recovery_complete and on the replica on
receipt of the BackfillComplete progress message), or upon leaving Active or
ReplicaActive.

It's important that we always grab the local reservation before the remote
reservation in order to prevent a circular dependency.