Decouple clean up ingest temp data from `BackendCtxMgr`

[tangenta]

Enhancement

Use a separate loop for cleaning up is better. Otherwise, if there are no new add index jobs running, temp-dir cannot be cleaned up.

[lance6716]

About the problem "if there are no new add index jobs running, temp-dir cannot be cleaned up.", I agree we should use a background loop to periodically check temp-dir. However, the only ownership of temp-dir should still belong to BackendCtxMgr. In other words, only BackendCtxMgr creates and deletes the files under its workering directory which is temp-dr.

Currently my suggestion is spawn a goroutine that calls BackendCtxMgr.CleanUpXXX. And the caller passes processingJobsIDs as an argument to avoid BackendCtxMgr uses getProcessingJobIDs.

[tangenta]

@lance6716 I plan to remove BackendCtxMgr and let each backend context managed by ddl worker / distributed framework instead. This is one of the steps.

[lance6716]

@lance6716 I plan to remove BackendCtxMgr and let each backend context managed by ddl worker / distributed framework instead. This is one of the steps.

OK, I realized that clean up stale files is not a functionality of one backend context, it belongs to the "backend context manager" but we are going to remove this concept.