* 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
Arya