There is a deadlock situation when ud-mailgate gets a mail claiming to be from

There is a deadlock situation when ud-mailgate gets a mail claiming to be from
itself:
 - ud-mailgate opens and locks the replay cache
 - verification of the mail fails for whatever reason
 - a reply is sent (to itself)
 - exim tries to deliver the mail by directly calling ud-mailgate
 - ud-mailgate tries to acquire the lock -> deadlock
Fix this by changing when we open the replay cache, and unlock it as soon as we
are done.