Pull Metrics struct out of Slave class.

Review: https://reviews.apache.org/r/28058/
Added slave active field in state.json.

Review: https://reviews.apache.org/r/28260
Removed trailing diff/ from review line in commit message.

Review: https://reviews.apache.org/r/28205
Simplified label JSON modeling with JSON::Protobuf.

JSON fields doesn't need to be set explicitly, so label modeling can
be simplified with JSON::Protobuf.

Review: https://reviews.apache.org/r/28287
Allowed turning off network socket statistics collection.

Review: https://reviews.apache.org/r/28295
Added DiskInfo to the Resource protobuf.

Review: https://reviews.apache.org/r/27550
Fixed the tab issue in mesos.proto.
Removed unnecessary calls to Shutdown() in the reconciliation tests.

Review: https://reviews.apache.org/r/28265
Added a task reconciliation continuation for re-use in the Master.

Review: https://reviews.apache.org/r/28266
Updated killTask to perform reconciliation for unknown tasks.

Review: https://reviews.apache.org/r/28267
Changed master label test to strictly enforce format.

Review: https://reviews.apache.org/r/28293
Changed slave label test to strictly enforce format.

Review: https://reviews.apache.org/r/28294
Match future dispatch messages with type info.

Review: https://reviews.apache.org/r/28052
Fixed future dispatch in tests to match function type.

Review: https://reviews.apache.org/r/28081
Added option to force pull docker image.

Review: https://reviews.apache.org/r/28190
Fixed a few grammar mistakes in mesos.proto.
Silenced symbolic link to pre-commit in bootstrap.

Fixes MESOS-1960.

Review: https://reviews.apache.org/r/28303