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
Initial import for health check
Fixed Protobuf to JSON serialization of optional fields with default values.

Previously, only optional fields which where explicitly set would be serialized regardless whether default values was present.
This patch adds those fields explicitly.

Review: https://reviews.apache.org/r/21246
Cleaned up JSON work
Snapshot of current health check program.
Add task health check