Currently, the only exposed entry point into the ApplicationMailbox's configured
routing system is to call `route`, which performs a lot of work to fully
`process` inbound email. It'd be nice to have a way (e.g. in test) of checking
which mailbox an email would route to without necessarily processing it yet.
This also adds a new column, message_checksum, to the action_mailbox_inbound_emails table
for storing SHA1 digest of the email source. Additionally, it makes generating the missing
message id deterministic and adds a unique index on message_checksum and message_id to
detect duplicate emails.
