dm vdo thread-registry: rename all methods to reflect vdo-only use

Otherwise, uds_ prefix is misleading (vdo_ is the new catch-all for
code that is used by vdo-only or _both_ vdo and the indexer code).

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