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/testlogthrottling.cc

@@ -19,7 +19,7 @@
 #include <iostream>
 #include <pthread.h>
 #include <semaphore.h>
-#include <log_manager.h>
+#include <maxscale/log_manager.h>
 
 using std::cerr;
 using std::cout;