even if the `_converse.bosh_session` instance does not exist.
We do this to avoid trying to reconnect with stale tokens upon
reconnection. Especially relevant for anonymous connections, but could
also be applicable when switching between websocket and BOSH connections
when reconnecting.
Also renamed `startNewBOSHSession` to `startNewPreboundBOSHSession`
- Remove the `keepalive` configuration setting. It is now always implicitly `true`.
- Remove the `expose_rid_and_sid` configuration setting.
- A `prebind_url` is now mandatory when setting `authentication` to `prebind`.
- It's no longer possible to pass in `rid` and `sid` values to `converse.initialize.