cam: Add a number of asserts to catch bad transactions

Ensure that we're in the right state / priority for each of the states
in the driver. These asserts assured that a prior patch that I committed
to fix a priority leak worked when a drive departed (and bounced back
too!). These have been running in our production since I committed the
change and haven't trigged.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D53259