Still lacks inter-tab communication to update state across tabs, i.e.
when sending a 1-on-1 message in one tab, it doesn't appear in another,
because that information is not available via the websocket connection.
- Create a new `Connection` class that extends Strophe.Connection and
move related code from `converse-core.js` into this class.
- Store the session in localStorage when using a worker
- Move XEP-0156 code to connection.js
This allows us to initialize the connection without needing to know the
domain.
If a message comes in after joining the MUC but before the MAM messages
have been fetched, then converse-mam will query for MAM messages newer
than that message, causing an empty history (except for that one message).
