Force disable C++11 in libprocess for g++-4.7.

Review: https://reviews.apache.org/r/19335
Force disable C++11 in mesos for g++-4.7.

Review: https://reviews.apache.org/r/19336
Fixed a regression in the Master stats.json endpoint.

Review: https://reviews.apache.org/r/19493
Only use 50% of the memory for systems with less than 2GB of memory.

Review: https://reviews.apache.org/r/19312
Filter all message events in the Master when not elected.

Review: https://reviews.apache.org/r/19374
Added cache::erase and cache::size.

Review: https://reviews.apache.org/r/19375