Hide callback methods internally to caller

Put the callbacks that we use when calling into Twisted's IMAP APIs as
private to the method calling them. Thereby we don't expose them
publicly, and don't have to use some kind of naming scheme if we for
example would have multiple on_fetch methods.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Query status on specified mailbox

For some reason we're not using the mailbox argument when checking the
status of a mailbox. Instead we're using a constant string so that it
will check the inbox no matter what mailbox was specified.

Check the status of the specified mailbox.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Don't pass proto as argument to callbacks

We're passing self._proto as an argument to some of our callbacks. This
is not necessary since we always have access to it. Remove the
unnecessary argument where we're using it.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>