When rebuilding the Poller Cache from command line, allow it to be multi-threaded

[eschoeller]

I think it would be great to allow the rebuild_poller_cache.php to spawn multiple processes. Otherwise rebuilding the poller cache can take a very, very long time!

[cigamit]

It's a real ugly script. It does way more than it has to. But if you move a server and relocate a path, it's great in that it reconstructs everything. I'm good with parallelization.

[TheWitness]

This is resolved now. Thanks @xmacan