api keys: add API key usage tracking.

Track when an API key was last used, which IP address last used it, and
how many times it's been used overall.

This is so you can tell when an API key was last used, so you know if
the key is safe to delete, and so you can tell if an unrecognized IP has
used your key.

CHANGELOG.md

@@ -3,6 +3,8 @@
 ### API Changes
 
 * You can now have multiple API keys.
+* You can now see when your API keys were last used, how many times they've
+  been used, and which IP address last used them.
 * API keys can be restricted to only work with certain IPs or certain API
   endpoints.
 * If you're an app or script developer, and you have an app that requests API