mgr / volumes: purge queue for async subvolume delete

mgr / volumes: purge queue for async subvolume delete

Support asynchronous subvolume deletes by handing off the delete
operation to a dedicated set of threads. A subvolume delete operation
renames the subvolume (subdirectory) to a unique trash path entry
and signals the set of worker threads to pick up entries from the
trash directory for background removal.

This commit implements a `thread pool` strategy as a class mixin.

Signed-off-by: Venky Shankar <vshankar@redhat.com>