a single executor converges and maintains one lock

this replaces the old behavior of (acquire lock -> wait -> repeat), which was
complicated by the fact that the lock's TTL wouldn't line up with the
executor's interval.

[finishes #66548086]

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>
bump deps

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>