KILL [CONNECTION | QUERY] support, part1

Preparation for adding KILL syntax support.
Session id changed to uint32 everywhere. Added atomic op.
Session id can be acquired before session_alloc().
Added session_alloc_with_id(), which is given a session id number.
Worker object has a session_id->SESSION* mapping, not used yet.

server/core/atomic.cc

@@ -26,6 +26,15 @@ int atomic_add(int *variable, int value)
 #endif
 }
 
+uint32_t atomic_add_uint32(uint32_t *variable, int32_t value)
+{
+#ifdef MXS_USE_ATOMIC_BUILTINS
+    return __atomic_fetch_add(variable, value, __ATOMIC_SEQ_CST);
+#else
+    return __sync_fetch_and_add(variable, value);
+#endif
+}
+
 int64_t atomic_add_int64(int64_t *variable, int64_t value)
 {
 #ifdef MXS_USE_ATOMIC_BUILTINS