imapservice.js

Rather than locking for just one message, lock for the entire command
when potentially updating Seen data.  With this, we can block all
connections the user has except the currently executing one, and
not need to read/write the seen data between every message.

While this can take minutes on very large subs, it's certainly
better than hours as previously.

It's rude to have multiple sockets actively pumelling the server
anyway.