osd: let pgs process map advances before booting

The OSD deliberate consumes and processes most OSDMaps from while it
was down before it marks itself up, as this is can be slow.  The new
threading code does this asynchronously in peering_wq, though, and
does not let it drain before booting the OSD.  The OSD can get into
a situation where it marks itself up but is not responsive or useful
because of the backlog, and only makes the situation works by
generating more osdmaps as result.

Fix this by calling activate_map() even when booting, and when booting
draining the peering_wq on each call.  This is harmless since we are
not yet processing actual ops; we only need to be async when active.

Fixes: #3714
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0bfad8ef2040a0dd4a0dc1d3abf3ab5b2019d179)
osd: move common active vs booting code into consume_map

Push osdmaps to PGs in separate method from activate_map() (whose name
is becoming less and less accurate).

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit a32d6c5dca081dcd8266f4ab51581ed6b2755685)