* Create `PushRequest` in push history DB: this will be used to store requests for push notifications, either pending or completed ones.
* Rename `WorkManagerRequest` to `WorkManagerRequestBuilder`: make its `build` method return a list of `WorkManagerRequestWrapper`, which can be used to enqueue normal or unique workers.
* Rename `PerformDatabaseVacuumRequestBuilder` and adapt it to the new API.
* Adjust other components using `WorkManagerRequest`.
* Replace `SyncNotificationWorkManagerRequestBuilder` with `SyncPendingNotificationsRequestBuilder` and `FetchNotificationsWorker` with `FetchPendingNotificationsWorker`: this new pair of request builder and worker allow enqueuing requests for a session id and, once the worker runs, retrieve all the pending request data and use it to fetch the associated events. This simplifies quite a bit how this data had to be passed or grouped, since it's no longer necessary to do so
* Add new methods to `PushHistoryService` to modify the `PushDatabase`:
- insertOrUpdatePushRequest
- insertOrUpdatePushRequests
- getPendingPushRequests
- removeOldPushRequests
* Make `PushHandler` just handle incoming pushes: those will be inserted into the pending push request table in DB, then handled by the new worker. Once the process finished, a new `NotificationResultProcessor` will handle the results and what needs to be done with them (call ringing, displaying notifications, etc.)
* Add `requestType` optional parameter to `WorkManagerScheduler.cancel` so we can decide to only cancel some kinds of requests.
* Add migration to remove existing work manager requests for fetching notifications, since the previous worker class no longer exists.
