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.
```
rails/actionmailbox$ bundle exec rake tests
(snip)
...
rails/activerecord/lib/active_record/persistence.rb:48: warning: in `create!':
the last argument was passed as a single Hash
rails/actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb:21:
warning: although a splat keyword arguments here
...
(snip)
```
