Co-Authors Plus: Improve performance of auto-suggest user search

[danielbachhuber]

Currently, we ​use and filter get_users() to handle the auto-suggest search. This works fine, but it would be better to improve its performance. We're doing queries every time the AJAX is fired.

One way we might do this is to load all of the users into cache on load and then search through that cached value with PHP instead. This way, we only need to to query once (on the first ajax search) and all other results are returned from the cached values.

[danielbachhuber]

Closing in favor of Github issues: ​https://github.com/danielbachhuber/Co-Authors-Plus/issues/5