app_voicemail: Remove dependency on the stasis cache

app_voicemail utilized the stasis cache when polling mailboxes for MWI. This
caused a memory leak (items were not being appropriately removed from the
cache), and subsequent slowdown in system processing. This patch removes the
stasis cache dependency, thus alleviating the memory leak. It does this by
utilizing the new MWI API that better manages state lifetime.

ASTERISK-28443
ASTERISK-27121

Change-Id: Ie89fedaca81ea1fd03d150d9d3a1ef3d53740e46