server: Add zmq based event and data interface to the server

To allow easily extracting or streaming the data to an external
analysis system, zeromq can be configured (and reconfigured). The
system works as fire and forget and no loss detection is present.

A simple go based client application is provided to subscribe to
the publisher.

Change-Id: I4f3e6d675023a81b7d2ee19bf1f44a2be0ca003c
diff --git a/configure.ac b/configure.ac
index 6e85db4..35b4ee0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -47,7 +47,7 @@
 PKG_CHECK_MODULES(LIBOSMOCORE, libosmocore >= 0.3.2)
 PKG_CHECK_MODULES(LIBOSMOGSM, libosmogsm >= 0.3.2)
 PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.3.0)
-PKG_CHECK_MODULES(LIBZMQ, libzmq >= 4.0.0)
+PKG_CHECK_MODULES(LIBZMQ, libzmq >= 3.2.2)
 
 
 # Coverage build taken from WebKit's configure.in