acquire maintenance lock when running incremental RPZ updates

acquire maintenance lock when running incremental RPZ updates

this addresses a race that could occur during shutdown or when
reconfiguring to remove RPZ zones.

this change should ensure that the rpzs structure and the incremental
updates don't interfere with each other: rpzs->zones entries cannot
be set to NULL while an update quantum is running, and the
task should be destroyed and its queue purged so that no subsequent
quanta will run.

(cherry picked from commit 286e8cd7ea887e82e45b3a3cae43b2c97c8dce56)