Introduce ProfilerOutputStats

Introduce ProfilerOutputStats

* Associate Profiler objects with a request context by adding a $context
  property with a getter and a setter.
* Introduce ProfilerOutputStats, which writes profiling data to the stats
  buffer associated with the current request context.
* Make it the Profiler class's responsibility to enforce $wgProfilerLimit.
* Deprecate $wgProfilerLimit in favor of the (more aptly named, IMO)
  $wgProfiler['threshold'] config setting.
* Tidy up Profiler instance creation code in Profiler::instance().
* Add Profiler::getOutputs, which returns an array of ProfilerOutput instances
  which are configured for the current profiler and whose canUse() method
  returns true.
* Make ProfilerStub not log by creating a stub ProfilerStub::logData() method
  which does not call the parent. Previously the parent class checked if $this
  was an instance of ProfilerStub and returned early if so.

Task: T90623
Task: T85641
Change-Id: Icf644ad3435c1f30d0a49957a97b481808a3153d