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
Introduce ClangFormat to Mesos.

Review: https://reviews.apache.org/r/26069