Depend on latest backbone.browserStorage which has support for IndexedDB
via localforage.

Storage operations are now asynchronous and transactional.

Bugs fixed (mostly by waiting for operations to complete):

* Rooms are now fetched asynchronously, so wait before triggering `show`
  or when closing.
* Make sure chat create/update transactions complete before firing events
* Make sure chats and messages have been fetched before creating new ones.
* When doing a `fetch` with `wait: false` on a collection and then
  creating a model in that collection, then once the read
  operation finishes (after creating the model), the collection is emptied again.
* Patch and wait when saving.
  Otherwise we have previously set attributes overriding later ones.
* Make sure api.roomviews.close returns a promise

Test fixes:

* Chats are now asynchronously returned, so we need to use `await`
* Wait for the storage transaction to complete when creating and updating messages
* Wait for all chatboxes to close
    Otherwise we get sessionStorage inconsistencies due to the async nature of localforage.
* Wait for room views to close in spec/chatroom.js

In the process, remove the `closeAllChatBoxes` override in
converse-controlbox by letting the `close` method decide whether it
should be closed or not.

- Increase z-index for chatbox flyouts so that they appear above the controlbox
- Also fix a margin offset bug for the controlbox (in smaller viewports)

Also move some other methods out of `converse-core` into the plugins
that use them.

- Always try to first get local cache before requesting the roster.
- Rename `roster_fetched` with `roster_cached`

Doesn't appear to be very useful. IIRC it was added as a workaround for
slow roster issues.

This was used for tests but is no longer necessary.

- Inform the user when fetching of bookmarks times out.
- Only set the cached flag if the user doesn't have bookmarks.

This is in anticipation of the localforage/IndexDB merge

They aren't used anymore.

Only XML is supported for now.

since we can't assume it's always available.

Fixes #1691