convert class methods to instance methods for IncomingMessageProcessor

fixes CNVS-12129

test plan
- regression on incoming mail

Change-Id: Ia9ab3419201c9fdbd89e2483a3fde51f54c7f982
Reviewed-on: https://gerrit.instructure.com/32594
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>