Fixed a multi-threaded race condition that could happen when communication...

Fixed a multi-threaded race condition that could happen when communication classes are shutting down. We currently don't protect communication connection classes against multi-threaded access. The connection is stored in the lldb_private::Communication.m_connection_ap auto_ptr member. We either need to add protections when accessing this class or not let anything racy occur. With this fix, we are doing the latter.

llvm-svn: 121647