Refactored libprocess Help to reduce compile time.

Reduces compile time overall by ~1/3rd. Move implementation of
libprocess help into cpp file.  Use strings::join to refactor macro
expansion that was injecting lots of overhead even when the expanded
functions were not used.

Review: https://reviews.apache.org/r/25798
Refactored libprocess class ProcessReference into its own header.

Move class ProcessReference out of process.cpp and into its own header.
Part of refactoring process.cpp.

Review: https://reviews.apache.org/r/25868