MXS-2710: Move client_count handling into client DCB

Due to the fact that both client connections and listeners use sessions in
2.3, the client_count tracking must be done inside the client DCB. In
addition to this, the max_connections check didn't take the current
pending connection into account which caused an off-by-one error.

This commit fixes the connection_limit test failure that was introduced by
commit 6306519e5e75575ba083ee2f0edfe7e624da5d26.

server/core/test/test_poll.cc

@@ -53,6 +53,7 @@ static int test1()
 
     SERVICE service;
     service.routerModule = (char*)"required by a check in dcb.cc";
+    dummy.service = &service;
 
     /* Poll tests */
     fprintf(stderr,