modules: update value_pairs_foreach() invocation to match 3.6 prototype

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
modules: add explicit cfg parameter to drivers

syslog-ng 3.6 changed the way drivers may reference the configuration
being parsed. Earlier it used the "configuration" global, now that's gone,
requiring changes in plugins that used the global earlier.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
modules: use stats/stats.h instead of stats.h

As syslog-ng changed this header's location.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
update: Updated modules to be syslog-ng 3.6 compatible
kafka: Updated kafka module to use 3.6 interface.
configure: Now incubator needs at least syslog-ng 3.6
ZMQ: Added zmq source, destination and transport
ZMQ: Make useage of zmq automatic

 * It means if user didn't set --enable-zmq,
   but zmq is installed it will compile with zmq
ZMQ: removed unused zmq_install.sh

 * It doesn't belong to incubator. It needed for travis only.
ZMQ: fixed zmq libs and cflags
ZMQ: Fixed segfault during reader deinit
ZMQ: Followed up 3.6 related changes

 * stats and other headers location
 * transport init_instance instead of init_method
 * GlobalConfig handling
 * LogThreadedDestination insert handling