Replace the redis-intensive LREM in #destroy.

LREM is O(N), and thus unsafe to use on enormous queues.
Use the same (slower but safer) logic as when removing all jobs for a class.

args is first pushed through a JSON round-trip to preserve the friendly
symbol.to_s semantics.
Merge pull request #919 from nevans/safe_job_destroy

Replace the redis-intensive LREM in #destroy.