* update scan task to notify subscribe caller once the new subscribed_keys are sent on the watch channel
* Fixes timing bug in scan gRPC method:
Joins register/subscribe scan service calls, sends SubscribeResults request first, and filters out duplicate results from channel
* Removes outdated TODO
* wraps subscribed_keys in an Arc before sending to watch channel, fixes typo
* Remove result senders for keys that have been removed
* processes SubscribeResults messages
* send tx ids of results to the subscribe channel
* replaces BoxError with Report in scan_range
* adds a watch channel for using subscribed_keys in scan_range
* updates args to process_messages in test
* adds a `subscribe` method to ScanTask for sending a SubscribeResults cmd
* updates test for process_messages to cover subscribe cmds
* impls SubscribeResult service request and updates sender type
* adds test for SubscribeResults scan service request
* adds acceptance test
* updates tests and imports
* fixes acceptance test by using spawn_blocking to avoid blocking async executor and setting an appropriate start height
* fixes test
* Applies suggestions from code review.
* use tokio mpsc channel in scan task instead of std/blocking mpsc
* use tokio mpsc channel for results sender
* adds `was_parsed_keys_empty` instead of checking that all the parsed keys are new keys
* fixes test failures related to send errors in scan task
* returns height and key for scan results from subcribe_results results receiver
* hide scan_service mod in zebra-node-service behind feature