osd: manage backoffs per-pg; drop special split logic

osd: manage backoffs per-pg; drop special split logic

Switch backoffs to be owned by a specific spg_t.  Instead of wonky split
logic, just clear them.  This is mostly just for convenience; we could
conceivably only clear the range belonging to children (just to stay
tidy--we'll never get a request in that range) but why bother.

The full pg backoffs are still defined by the range for the pg, although
it's a bit redundant--we could just as easily do [min,max).  This way we
get readable hobject ranges in the messages that go by without having to
map to/from pgids.

Add Session::add_backoff() helper to keep Session internals out of PG.h.

Signed-off-by: Sage Weil <sage@redhat.com>