Added process ID prefixes for ZooKeeper related libprocess Processes which are helpful for debugging.

Review: https://reviews.apache.org/r/22161
Fixed memory leaks and cleaned up the mesos containerizer logic.

Review: https://reviews.apache.org/r/22159
Updated dropped message logging to use VLOG.

Review: https://reviews.apache.org/r/22164
Added a concurrent 'Queue' to libprocess.

Review: https://reviews.apache.org/r/20308
Introduced a low-level scheduler API.

Review: https://reviews.apache.org/r/20309
Removed unsignedness from constant.
Removed newline (style failure).
Increased the registry store timeout for tests.
Fixed a segfault in libprocess.

Review: https://reviews.apache.org/r/22247
Updated CHANGELOG for 0.19.0.
Validated Call's passed to Mesos::send.
Pulled common Framework (re-)registration validation into a helper method.

Review: https://reviews.apache.org/r/22162
Prevented slave from (re-)registering when TERMINATING.

Checked the slave's state before (re-)registering.
Introduced EXPECT_NO_FUTURE_PROTOBUFS() and EXPECT_NO_FUTURE_MESSAGES().

Review: https://reviews.apache.org/r/21968
Fixed style glitch in slave tests.

Fixed a style error.

Review: https://reviews.apache.org/r/22255
Added an abstraction for launching operations in a subprocess.

Review: https://reviews.apache.org/r/22224
Added setns utilities to stout.

Review: https://reviews.apache.org/r/22160
Added virtual destructor to Operation base class.

Review: https://reviews.apache.org/r/22311
Added terminal action to routing lib.

Review: https://reviews.apache.org/r/22310
Allowed net::IP to be created from a dot decimal string.

Review: https://reviews.apache.org/r/22294
Used the new net::IP interfaces.

Review: https://reviews.apache.org/r/22295