`Worker.Hello()`: Fix a `forceReset` regression.

As of #65, `{Emcee, No}Store.Exists()` correctly returns `false` if a
device ID has not previously registered with the push server. This
change caused `Worker.Hello()` to reset all client-specified IDs.

The client will now be assigned a new ID under the following
conditions:

* An explicit ID is not specified (i.e., the `"uaid"` field is `""` or
  omitted entirely).
* A client with the specified ID is already connected.
* The number of channels in the opening handshake exceeds the number of
  subscriptions allowed by the backing store.
* The client specifies existing channels for a nonexistent ID.
Expand `TestHeloInvalidIds()`.
Merge pull request #80 from mozilla-services/bug/force-reset

Fix a `forceReset` regression