ceph_test_objectstore: add AppendWalVsTailCache test

ceph_test_objectstore: add AppendWalVsTailCache test

This specifically exercises a WAL overwrite racing with a subsequent
append that uses the previous writes cached tail.  We need to make sure
the use of the cached_tail waits for the previous WAL write or else our
update of tail block will be overwritten with the prior (WAL) value.

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