feat(queued-request-controller): Batch RPC requests by origin

The `QueuedRequestController` will now batch RPC requests by origin.
Each batch of requests will be processed in parallel, allowing existing
features relating to groups of requests to function correctly with
queuing enabled (e.g. the confirmation navigation buttons, and the
"Reject all" button). The batching by origin also ensures that
confirmations from different dapps are never displayed together in a
group, which will help prevent users from confusing which dapp each
confirmation is from.

The meaning of `queuedRequestCount` has changed; it now represents the
total number of queued requests, i.e. those that are queued but are
**not** being processed. Previously it included processing requests as
well. Given that the number of confirmations awaiting approval is
already tracked elsewhere, it was not useful to double count them by
including them in the queued request count as well.

Closes #3763