(PUP-2747) Add support for multiple profilers

This commit makes a slight modification to the profiling system,
such that we can register multiple profilers at the same time.

This allows callers to register their own profiler without
worrying about collisions with profilers that might have
been registered by other parts of the code.
(maint) Stop using ambiguous Manager name

Manager is not a very good name for a class. What does it manage? What
does it mean to manage that? How does it manage? Who manages the
manager? This renames the Profiler::Manager class to just
Profiler::AroundProfiler because it modifies profilers to work so that
they automatically go around a block. This is then the interface exposed
by the Puppet::Util::Profiler singleton.
Merge branch 'pr/2758'

* pr/2758:
  (maint) Stop using ambiguous Manager name
  (PUP-2747) Add support for multiple profilers