MXS-1378 Provide access to current session

Provide access to current session and session id. This will
be used by the logging mechanism for logging the session id
together with messages.

server/core/session.c

@@ -960,3 +960,17 @@ bool session_valid_for_pool(const MXS_SESSION* session)
     ss_dassert(session->state != SESSION_STATE_DUMMY);
     return session->qualifies_for_pooling;
 }
+
+MXS_SESSION* session_get_current()
+{
+    DCB* dcb = dcb_get_current();
+
+    return dcb ? dcb->session : NULL;
+}
+
+uint64_t session_get_current_id()
+{
+    MXS_SESSION* session = session_get_current();
+
+    return session ? session->ses_id : 0;
+}