Fix bug #303.

Fix bug #303.

When failover is triggered by worker process, it is possible that
wrong DB node could failover. This is due to the db_node_id member in
the POLL_CONNECTION structure is not initialized in the process (in
child process the member is properly initialized). To solve the
problem, add new function pool_set_db_node_id() to set the structure
member variable and call it inside
make_persistent_db_connection(). For this purpose also the new
parameter "db_node_id" is added to make_persistent_db_connection and
friends.