Fix ProcTest.MultipleThreads hanging.

Replace 'sleep' with 'pthread_testcancel'.

Review: https://reviews.apache.org/r/28137
Fix a race between SocketManager link() and send().

Review: https://reviews.apache.org/r/27967
Factor out Future callback invocation.

Factor out callback invocation in Future to make the logic easier to
read. It also de-duplicates some code.

Review: https://reviews.apache.org/r/28195
Replace std::queue with std::vector in Libprocess Future.

Use std::vector instead of std::queue to reduce dynamic allocations
and improve performance.

Review: https://reviews.apache.org/r/28196
Do not return error if removing bind mount fails.

Review: https://reviews.apache.org/r/28339
Set memory::limit_in_bytes first, even with limitSwap enabled.

Review: https://reviews.apache.org/r/28305
Allow mesos containerizer to destroy correctly at each state.

Review: https://reviews.apache.org/r/28141