Previously, a client disconnecting while a sample was playing could
lead to issues. For example, if a client disconnected before the
"ready" signal of the sample-play arrives, `operation_new_cb()`
would be called and that would try to use the client's pw_manager,
however, that has previously been destroyed in `client_disconnect()`.
If the client disconnected after the "ready" signal but before the reply
has been sent, then `sample_play_ready_reply()` would never be called
since operations are completed via the client's pw_manager which
would already be destroyed at that point.
Fix this by installing a listener on the client, and properly
cancelling the operation and making sure that the pending_sample
is correctly destroyed.
Wait the reply to be sent and the sample to complete playback before
freeing the sample. Otherwise it might have been possible that the
sample completes before the reply can be sent.
