Add ContainerInfo and Volumes protobufs.

Review: https://reviews.apache.org/r/24349
Made value in CommandInfo optional and added argv.

Review: https://reviews.apache.org/r/24619
Refactored the protobuf message comparison logic.

Review: https://reviews.apache.org/r/24631
Updated health check to use the new CommandInfo.

Review: https://reviews.apache.org/r/24632
Updated mesos containerizer launcher to use the new CommandInfo.

Review: https://reviews.apache.org/r/24633
Updated slave http to use the new CommmandInfo.

Review: https://reviews.apache.org/r/24634
Updated command executor to use the new CommandInfo.

Review: https://reviews.apache.org/r/24635
Fixed bug in io::poll.

When a future returned from process::io::poll is discarded we need to
fully remove the ev_io watchers from libev otherwise the reuse of
those file descriptors can cause untended side effects because they're
still being used by the kernel/libev.

Review: https://reviews.apache.org/r/24194
Revert "Refactored the protobuf message comparison logic."
This reverts commit 2d4eb0ef3c162502595f135408b0fe42cf29ebfa.

The original equality check does not care about the order of repeated
fields (e.g., environments).

Review: https://reviews.apache.org/r/24657
Updated equality check for CommandInfo.

Review: https://reviews.apache.org/r/24658
Removed unused test file 'process_spawn.cpp'.

Review: https://reviews.apache.org/r/24582
Added a missing test target in Makefile.am.

Review: https://reviews.apache.org/r/24583
Some Master cleanups.

Review: https://reviews.apache.org/r/24576
Model pending tasks in the Master's metrics and JSON.

Review: https://reviews.apache.org/r/24515
Expose pending tasks during reconciliation.

Review: https://reviews.apache.org/r/24516
Fixed a compiler warning in values_tests.cpp.
Fixed a resource leak in ThreadLocal.

Review: https://reviews.apache.org/r/24669
Renamed argv in CommandInfo to arguments.

Review: https://reviews.apache.org/r/24672
 Added GLOG signal handler to print stack traces on segfault in stout.

Review: https://reviews.apache.org/r/24675
Added GLOG signal handler to print stack traces on segfault in
libprocess.

Review: https://reviews.apache.org/r/24674