Added UNREACHABLE to Clock::settled to silence compiler warning.
Fix compilation error for Socket on older linux + OSX.

Test whether SOCK_NONBLOCK and SOCK_CLOEXEC are defined to use single system call to create socket.
Otherwise use os::nonblock and os::cloexec after creating the socket.

Review: https://reviews.apache.org/r/28125
Refactoring fetcher code a bit more (MESOS-2056) in preparation of fetcher cache

As a follow up to MESOS-1316, moved all fetcher run code into fetcher name space.

Review: https://reviews.apache.org/r/27995
Match future dispatch messages with type info.

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

Review: https://reviews.apache.org/r/28081
Allow mesos containerizer to destroy correctly at each state.