Added xdebug tracing/profiling skeleton

Summary: This diff adds the backend for tracing, profiling, and stack trace printing.

Each of these xdebug categories requires some sort of tracing
information to be stored, but all three can be enabled at once so they must
be implemented as a single profiler/tracer (as we can only have one profiler
attached at a time per thread).

I've implemented a class XDebugProfiler that extends from Profiler in
ext_hotprofiler.h and I used a similar approach as the TraceProfiler in
ext_hotprofiler.cpp.

There are a few inconsistencies with php5 xdebug that I've documented.
Namely, by default no profiler is required for basic xdebug functionality.

Some of the remaining functionality depends on xdebug variable serializing
implemented in D1412665.

Depends on D1433761

Reviewed By: @ptarjan

Differential Revision: D1423763