Creates a socket host object.

This patch moves the logic of many common socket operations into
its own class lldb_private::Socket.  It then modifies the
ConnectionFileDescriptor class, and a few users of that class,
to use this new Socket class instead of hardcoding socket logic
directly.

Finally, this patch creates a common interface called IOObject for
any objects that support reading and writing, so that endpoints
such as sockets and files can be treated the same.

Differential Revision: http://reviews.llvm.org/D4641

Reviewed by: Todd Fiala, Greg Clayton

llvm-svn: 214984