Added variadic strings join.

There is a second version of the variadic join which takes a reference
to a stringstream as a parameter. This is handy when strings::join is
just a part of a larger string manipulation.

Review: https://reviews.apache.org/r/25789
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
Updated semantics of disconnected/deactivated slaves/frameworks in
master.

Review: https://reviews.apache.org/r/25866
Updated ping message to embed the slave connected status.

Review: https://reviews.apache.org/r/25867
Added reconcileTasks to python scheduler.

The last step of wiring up reconcileTasks in the python bindings.

Review: https://reviews.apache.org/r/25986