Introduces task manager's compaction module. That's an initial
part of integration of compaction with task manager.
When fully integrated, task manager will allow user to track compaction
operations, check status and progress of each individual one. It will help
with creating an asynchronous version of rest api that forces any compaction.
Currently, users can see with /task_manager/list_modules api call that
compaction is one of the modules accessible through task manager.
They won't get any additional information though, since compaction
tasks are not created yet.
A shared_ptr to compaction module is kept in compaction manager.
Closes#12635
* github.com:scylladb/scylladb:
compaction: test: pass task_manager to compaction_manager in test environment
compaction: create and register task manager's module for compaction
tasks: add task_manager constructor without arguments
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