Do service client counts in the core

As all connections should be accepted via dcb_accept, it is the optimal
place to calculate how many open client connections per service there
are. The decrementation should be done when the session is closed instead
of when the call to dcb_close for the client DCB is done. This allows the
client count to be the absolute reference count that sessions have to a
service.

The current client count is a duplicate counter that should match the
n_current value in SERVICE_STATS. The former does differ from the latter
in that it does the incrementation when the client DCB is accepted instead
of when the session is created.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1418,7 +1418,6 @@ static void gw_process_one_new_client(DCB *client_dcb)
     CHK_PROTOCOL(protocol);
     client_dcb->protocol = protocol;
 
-    atomic_add(&client_dcb->service->client_count, 1);
     //send handshake to the client_dcb
     MySQLSendHandshake(client_dcb);