Add MessageQueue class

MessageQueue encapsulates a message queue built on top of a
pipe. The message queue needs a handler for receiving messages
and must be added to a worker for pumping messages through the
pipe.

Each Worker will have an instance of MessageQueue.

server/core/CMakeLists.txt

@@ -18,6 +18,7 @@ add_library(maxscale-common SHARED
   maxscale_pcre2.cc
   misc.cc
   mlist.cc
+  messagequeue.cc
   modulecmd.cc
   modutil.cc
   monitor.cc