crush/mapper: drop confusing const for descend_once

Signed-off-by: Sage Weil <sage@inktank.com>
crush/mapper: add support for descend_once == 2

Try to move past the not-quite-right behavior of the descend_once
tunable (wherein it will not stop on a collision).  Make the new
behavior identical to what you get from the 'indep' buckets.

Signed-off-by: Sage Weil <sage@inktank.com>
crush/CrushWrapper: make descend_once==2 the new optimal

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: allow setting 'firefly' tunables

Signed-off-by: Sage Weil <sage@inktank.com>
osd/OSDMap: add feature bit for TUNABLES3 (== CRUSH_V2)

Add a feature bit for the new tunables.  Re-use the same feature bit
as CRUSH_V2 because the support is part of the same code update.

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