Revert "os/bluestore: allow multiple DeferredBatches in flight at once"

Revert "os/bluestore: allow multiple DeferredBatches in flight at once"

This reverts commit ca32d575eb2673737198a63643d5d1923151eba3.

If we have multiple batches in flight then we have to worry about writes
to the same blocks reordering.

Also, 3c6a6c46d5808d6c42ed4dcfb441bad64366686b is sufficient to avoid the
stall/deadlock in http://tracker.ceph.com/issues/20295.

# Conflicts:
#	src/os/bluestore/BlueStore.cc

Fixes: http://tracker.ceph.com/issues/20925
Signed-off-by: Sage Weil <sage@redhat.com>