os/bluestore: fix objectstore_blackhole read-after-write

os/bluestore: fix objectstore_blackhole read-after-write

Instead of tossing out transactions from queue_transactions (which prevents
us from reading back the results), instead (1) stop doing any write IO, and
(2) stop trimming the cache.

Fixes: https://tracker.ceph.com/issues/40684
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 6c2a8e472dc71b962d7de008e30631f125b148c3)

 Conflicts:
	src/os/bluestore/BlueStore.h
   Due to cache refactoring one should handle objectstore_blackhole param
   inside Cache:trim/trim_all funcs.