Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

server/core/test/testmemlog.c

@@ -33,7 +33,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
-#include <memlog.h>
+#include <maxscale/memlog.h>
 
 /**
  * Count the number of lines in a file