Last seen update for MySQL storage backend

[jessepeterson]

Create a last_seen_at column in the MySQL schema and update it (nearly) every time the client talks to us.

[jessepeterson]

Note there's a few edge cases where we can't update the last seen column. Notably because Declarative Management isn't "in scope" for the server the storage layer has no methods for it and thus if the client tries to talk to us there the storage will be none of the wiser. This is the case in the NanoMDM storage. That said the "important" beats of enrollment and command result storage & retrieval are covered (which includes Idle responses to pushes and are likely the only actual thing we care about).

[kilpatds]

LGTM?