lei: generalize auxiliary WQ handling

lei: generalize auxiliary WQ handling

op_wait_event is now more lei-specific since we no longer have
to care about oneshot and use a synchronous loop.

{ikw} (import-keywords) started a trend, but LeiPmdir (parallel
Maildir) is an upcoming WQ class that will follow this idea.

Eventually, {l2m} usage may be updated to follow this, too.