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/modules/include/readwritesplit.h

@@ -24,8 +24,8 @@
  * @endverbatim
  */
 
-#include <dcb.h>
-#include <hashtable.h>
+#include <maxscale/dcb.h>
+#include <maxscale/hashtable.h>
 #include <math.h>
 
 #undef PREP_STMT_CACHING