Ceph: tune config knobs in response to mini-outage

gdb showed threads blocked on a pthread lock; further investigation
showed the thread pool has 100 max threads which is too few (it /might/
not be yet, but it will certainly be in the future).

out'ing two OSDs resulted in maxing out the GbE of two ms-be due to a
large number of backfills (256). Tune a couple of settings to sacrifice
recovery speed in favor of cluster stability.

Change-Id: I3945fff1cabfffde20519eed66670d62d89b2724