Currently, make_and_start_task returns a pointer to task_manager::task
that hides the implementation details. If we need to access
the implementation (e.g. because we want a task to "return" a value),
we need to make and start task step by step openly.
Return task_manager::task::impl from make_and_start_task. Use it
where possible.
Fixes: https://github.com/scylladb/scylladb/issues/22146.
Optimization; no backport
Closesscylladb/scylladb#25743
* github.com:scylladb/scylladb:
tasks: return task::impl from make_and_start_task
compaction: use current_task_type
repair: add new param to tablet_repair_task_impl
repair: add new params to shard_repair_task_impl
repair: pass argument by value
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