Assign hostgroup from previous COM_QUERY

The destination hostgroup assigned from previous COM_QUERY commands is
the one used to establish the fast_forward connection. As a result,
multiple binlog clients can consume binlog events from different MySQL
hostgroups using the same username.

include/MySQL_Session.h

@@ -246,6 +246,7 @@ class MySQL_Session
 	int user_max_connections;
 	int current_hostgroup;
 	int default_hostgroup;
+	int previous_hostgroup;
 	/**
 	 * @brief Charset directly specified by the client. Supplied and updated via 'HandshakeResponse'
 	 *   and 'COM_CHANGE_USER' packets.

lib/MySQL_Session.cpp

@@ -4010,6 +4010,7 @@ __get_pkts_from_client:
 								// The following code prepares the session as if it was configured with fast
 								// forward before receiving the command. This way the state machine will
 								// handle the command automatically.
+								current_hostgroup = previous_hostgroup;
 								mybe = find_or_create_backend(current_hostgroup); // set a backend
 								mybe->server_myds->reinit_queues(); // reinitialize the queues in the myds . By default, they are not active
 								// We reinitialize the 'wait_until' since this session shouldn't wait for processing as
@@ -7440,6 +7441,7 @@ void MySQL_Session::RequestEnd(MySQL_Data_Stream *myds) {
 		CurrentQuery.end();
 	}
 	started_sending_data_to_client=false;
+	previous_hostgroup = current_hostgroup;
 }