[MODCLUSTER-414] Fix compilation warnings in the native part - mod_proxy...

..._cluster part

* lines 958, 957, 677, 956: move several statements that are used only in \#ifdef-s
* line 2669: remove unused variable userdata_key
* line 1709: initialize variable
* line 1419: unformattable data passed to debug log

https://issues.jboss.org/browse/MODCLUSTER-414
Anything else we would like to log here?