add getAllInboxMessageIds and getAllSentMessageIds to API

Currently if you don't know the message id your messages, you must resort to calling getAllXMessages and crawl through the list.  Given a large inbox with some large messages, building the returning JSON string can consume all of your system's resources.

By providing the methods getAllInboxMessageIds and getAllSentMessageIds that return the Id's only, the consumer software can more intelligently decide what messages it needs to retrieve, and it can retrieve the messages individually, reducing server load.

Results could be simpler and return a list, but they're consistent with the other API methods that return more object-like formats:

    {"inboxMessageIds": [{ "msgid": "deadbeef"}, {...}, ...]}