mail: fixes of default notifier (reset)

This fixes POP3 notification messages.

Change-Id: Ic7d9c0bb23c4dc9814f6c4bade0aba367fa28db1
Reviewed-on: https://review.haiku-os.org/685
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Mail server: fix disabling notifications

* The default notifier didn't always take the setting into account.
* The mail server was not using the setting from the settings file and
instead waiting for a message that wasn't sent anywhere.

Fixes #10852.

Completed transition to the new mail API; ie. it compiles.

* Completely untested yet. Unlikely to work yet.

Minor MailNotifier cleanup.

* Renamed to BMailNotifier, as it's part of the public API.
* Renamed Notifier.{cpp|h} to DefaultNotifier.{cpp|h} as that's the class it
implements.
* Made the mail counts uint32, and the byte counts uint64.

Mail server: fix disabling notifications

* The default notifier didn't always take the setting into account.
* The mail server was not using the setting from the settings file and
instead waiting for a message that wasn't sent anywhere.

Fixes #10852.

Completed transition to the new mail API; ie. it compiles.

* Completely untested yet. Unlikely to work yet.

Minor MailNotifier cleanup.

* Renamed to BMailNotifier, as it's part of the public API.
* Renamed Notifier.{cpp|h} to DefaultNotifier.{cpp|h} as that's the class it
implements.
* Made the mail counts uint32, and the byte counts uint64.