Refactored the environment-based test filtering.

Review: https://reviews.apache.org/r/25569
Removed duplicate error messaging in Docker::create.
Added support for getting socket diagnosis information.

Review: https://reviews.apache.org/r/25948
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
Normalized and enforced 3rdparty/libprocess conditional include style.

Review: https://reviews.apache.org/r/26056
Dynamically change poll interval in the reaper.

If pid count <= 50 then use 100 ms (<= 0.5% core usage), if count
>= 500 use 1000 ms (<= 1% core usage at 500 pids), else interpolate.

Review: https://reviews.apache.org/r/25947
Adds The Factory to PoweredByMesos documentation.
Added docker container prefix to docker docs.

Review: https://reviews.apache.org/r/26052
Adds OpenCredo and WooRank to PoweredByMesos page.
Increased the store timeout for the Registrar tests.

Review: https://reviews.apache.org/r/26141
Switch [stout] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25191 (I originally authored the patch, but Patrick was managing it for various reasons). Incorporates the fix suggested by Ben Mahler. Fixes Dominic's build issue.

Review: https://reviews.apache.org/r/26100
Switch [libprocess] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25192

Review: https://reviews.apache.org/r/26101
Switch [mesos] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25193

Review: https://reviews.apache.org/r/26103
Fixed AllocatorTest.FrameworkExited test.

Review: https://reviews.apache.org/r/26144
Remove depreacted webui_hostname and webui_port fields from SlaveInfo.

These have been deprecated since November 2013, at least Mesos 0.16.
Should be safe to remove at this point (People will have to upgrade
pre-0.16 to something between 0.16 and 0.20.1 inclusive, before
upgrading past 0.20.1).

Review: https://reviews.apache.org/r/26149