dm vdo thread-utils: push uds_*_cond interface down to indexer

Only used by indexer components. Also return void from
uds_init_cond(), remove uds_destroy_cond(), and fix up
all callers.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>