Currently, all apis that start a compaction have two versions:
synchronous and asynchronous. They share most of the implementation,
but some checks and params have diverged.
Add consider_only_existing_data parameter to /tasks/compaction/keyspace_compaction/{keyspace},
to match the synchronous version of the api (/storage_service/keyspace_compaction/{keyspace}).
Unify the handlers of both apis.
For all compaction types which can be started with api, add an asynchronous
version of api, which returns task_id of the corresponding task manager
task. With the task_id a user can check task status, abort, or wait for it,
using task manager api.
