Requests for comment/Better PHP profiling

This RFC exists so we can investigate alternative profiling implementations to the one we have now.

This is a rough draft and shouldn't be taken seriously just yet.

Problems the DIY approach solves

 * 1) production debugging via ?forceprofile=1 and ?forcetrace=1

Problems the DIY approach doesn't solve

 * 1) DIY approach in MediaWiki is error-prone
 * 2) People forget to include closing profiling
 * 3) People have to remember to include profiling at all
 * 4) Things that might *need* profiling aren't
 * 5) DIY might be slower than something built into PHP

Solutions

 * xhprof
 * Works in both stock PHP as well as HHVM, which is nice
 * Seems...fast...and complete
 * xdebug?
 * I think it's slow, from the last time I tried