Potential segfault in `link` and `send` when linking to a remote process

There is a race in the SocketManager, between a remote link and disconnection of the underlying socket.

We potentially segfault here: https://github.com/apache/mesos/blob/215e79f571a989e998488077d713c28c7528926e/3rdparty/libprocess/src/process.cpp#L1512

*socket dereferences the shared pointer underpinning the Socket* object. However, the code above this line actually has ownership of the pointer:
https://github.com/apache/mesos/blob/215e79f571a989e998488077d713c28c7528926e/3rdparty/libprocess/src/process.cpp#L1494-L1499

If the socket dies during the link, the ignore_recv_data may delete the Socket underneath link:
https://github.com/apache/mesos/blob/215e79f571a989e998488077d713c28c7528926e/3rdparty/libprocess/src/process.cpp#L1399-L1411

The same race exists for send.

This race was discovered while running a new test in repetition:
https://reviews.apache.org/r/49175/

On OSX, I hit the race consistently every 500-800 repetitions:

3rdparty/libprocess/libprocess-tests --gtest_filter="ProcessRemoteLinkTest.RemoteLink"  --gtest_break_on_failure --gtest_repeat=1000