get_user_by_email() and get_user* cleanup

[ryan]

Add a get_user_by_email() function to get a user by email address. It should use an email to user id cache to avoid DB trips. If an email address does not match a user, cache that fact. If this results in too much stuff in the cache, we can lose this part.

While we're at it. Clean up the get_user* functions. Refactor the back compat and caching code into a new private function shared by all of the get functions.

[Bobcat]

How does this relate to the changes you requested in #4642 ? Patches there have functions get_userdatabyemail() or get_user_by(), which you requested.

[ryan]

This provides the same thing. I decided to go with get_user_by_email() instead of a generic get_user_by(). This patch also does some cleanup and cache work that I've been meaning to do.

[ryan]

(In [6346]) get_user_by_email() and some user caching cleanups. fixes #5392

[Bobcat]

So are you going to modify #4642 to work with your changes?