- Wrote more unit tests to make sure filestore behaves as expected
- Renamed NewFileDataStore to DefaultFileDataStore, implemented a new
NewFileDataStore for unit tests.
- filestore now removes entire mailbox dir when last message is deleted
- Refactor datastore such that we have a FileDataStore that implements
the DataStore interface.
- Add in missing SMTP configuration options: max recips, max idle, max
message size
- Add retention options to config