fast DDL sort path change to support multiple fast DDL jobs running simultaneously

[lance6716]

Enhancement

After walk the fast DDL sort path, there should be functionality that a) must remove all folders that won't be used in future, maybe caused by DDL owner switch and has finished this job. And b) optionally remove not running folders to release space for other tasks.

To support a), ddl can maintain a memory cache of all processing = 1 jobs of tidb_ddl_job table, if a folder's job ID does not exist in the cache, we can know it's finished by other DDL owner. The cache has some similarity with runningJobs which is the picked running job of this DDL owner, maybe we can reuse that structure.

To support b), we can reuse the runningJobs to know if the job is running. And limit the caller goroutine to not concurrently call b) and modify runningJobs

[lance6716]

/cc @tangenta