* merges scan task tests and checks that the scan task removes keys
* Adds test to CI
* Skip to expected block height and assert that the result is from that height
* Updates test docs and uses zebrad state path for scan cache
* Log "Zcash network: {network}" from scan task commands test
* 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
* moves ScanTask to its own module in service module
* moves `process_messages()` method to scan_task.rs
* uses get_mut() and returns new keys
* updates types and adds scan_until() function
* adds and uses wait_for_height() function
* spawns scan_until_task_handler to poll scan_until_tasks FuturesUnordered
* corrects comment
* adds TODO
* updates expected test log, corrects panic msg, fixes fmt
* moves scan functions to scan_task module
* moves ScanTaskCommand and related method impls to its own module
* moves `ScanTask::mock()` test constructor to its own module
* creates `add_keys` mod and moves `scan_until()` function there
* moves scan task executor to its own module and adds ScanRangeTaskBuilder type
* renames add_keys to scan_range, moves executor to scan_task mod
* adds test for process_messages
* updates scan_height_and_store_results() to skip last_scanned_height check if there is no key height for a key in `parsed_keys`
* updates `default_test_config()`
* adds acceptance test for registering keys in ScanTask
* uses the right feature for the new acceptance test
* Applies suggestions from code review