Support for mysql session variable commands, for example, SET AUTOCOMMIT=0

Session commands are identified by query clasisfier, and added to the session command property list in router client session object.
Session commands are then executed in existing backend servers but only one of them will reply to client.

server/include/spinlock.h

@@ -53,6 +53,8 @@ typedef struct spinlock {
 #define SPINLOCK_INIT { 0 }
 #endif
 
+#define SPINLOCK_IS_LOCKED(l) ((l)->lock != 0 ? true : false)
+	
 extern void	spinlock_init(SPINLOCK *lock);
 extern void	spinlock_acquire(SPINLOCK *lock);
 extern int	spinlock_acquire_nowait(SPINLOCK *lock);