Added a base PID set in log ZooKeeper network.

Review: https://reviews.apache.org/r/20602
Fixed implicit cast from double to size_t in statistics.

Review: https://reviews.apache.org/r/20614
Fixed MesosContainerizer to correctly update resources after recovery.

Review: https://reviews.apache.org/r/20505
Initialized MetricsProcess during libprocess initialization.

Review: https://reviews.apache.org/r/20615
Added a test for testing the base set in log ZooKeeper network.

Review: https://reviews.apache.org/r/20618
Fixed compiler error (gcc-4.1.2) in state tests.

Review: https://reviews.apache.org/r/20627