CLEANUP: lb-chash: free lb_nodes from chash's deinit(), not global

CLEANUP: lb-chash: free lb_nodes from chash's deinit(), not global

There's an ambuity on the ownership of lb_nodes in chash, it's allocated
by chash but freed by the server code in srv_free_params() from srv_drop()
upon deinit. Let's move this free() call to a chash-specific function
which will own the responsibility for doing this instead. Note that
the .server_deinit() callback is properly called both on proxy being
taken down and on server deletion.