Fix protobuf detection on systems with Python 3 as default

MESOS-1774

Review: https://reviews.apache.org/r/25439
Added kill() to freezerTimedOut() in cgroups.cpp.
This is a short-term fix for MESOS-1758.

Review: https://reviews.apache.org/r/25457
Fixed python egg proto imports.

Fixes defective import introduced by the flat folder hierachy of the mesos.interface Python egg generation.

Review: https://reviews.apache.org/r/25334
First try to fix #1736.
Second try to fix #1736.
Third try to fix #1736.
Third try to fix #1736. Merge with existed test.
Remove unused include.
First try to fix #1571. Propagate shutdown timeout to Executor and CommandExecutor via env variable. Remove escalation constant for command executor.
Second try to fix 1571.