The `tabId` of the message sender is now added to the middleware
request object. This step is omitted if the `tabId` is not provided.
This is done early in the middleware stack, so the `tabId` should be
available for any subsequent middleware to use.
The Onboarding Middleware has also been modified to get the `tabId`
from the request object, rather than expecting it as a parameter upon
creation.
This refactor will enable further uses of the `tabId`.
* Remove unused onboarding stream
* Pass `sender` through to `setupProviderEngine`
The Port `sender` has been passed down a few more layers. This allows
us to get more information from the sender deeper in the stack, but
also simplifies things a bit as well. For example, now the "fake"
URL object with the `metamask` hostname is no longer needed.
* Create onboarding middleware
This middleware intercepts `wallet_registerOnboarding` RPC messages. It
will register the sender as an oboarding initiator if possible, and
otherwise ignores the message.
