Convert XMLReader to HNI

Summary: fixes #3972

Reviewed By: @ptarjan

Differential Revision: D1615944
Increase JitPGOThreshold to 100

Summary: Although I wanted to have a higher threshold to allow collecting more
profiling information before producing optimized translations, we
initially settled with a threshold of only 10 based on perflab
experiments.  Now that perflab got smarter about HHVM warmup, it seems
to like 100 better.  (I also tried bumping this up to 1000, but that
didn't seem to provide further wins.)

Reviewed By: @swtaarrs, @alexmalyshev

Differential Revision: D1635551