`local_ltiprovider_cron` needs optimizing

[hello-josh]

There are items being checked in a loop that could be filtered in the select statements for users. For instance, serviceurl and sourceid being empty [1] could be filtered in the user select [2]. Also multiple field updates should be handled in a single update statement [3]

[1] https://github.com/jleyva/moodle-local_ltiprovider/blob/master/lib.php#L234
[2] https://github.com/jleyva/moodle-local_ltiprovider/blob/master/lib.php#L230
[3] https://github.com/jleyva/moodle-local_ltiprovider/blob/master/lib.php#L294

Otherwise the processing of cron becomes exponentially slower as you expose more items via lti provider. Thanks!

[jellisii]

I've actually worked around this by using the event system. I've only tested my modifications to the plugin on Moodle V2.9 though.