Store a pointer to the owner `impl` object instead of
the (embedded) `stat` object. This way `message_free()`
can be simplified since the owner `impl` does not need
to be passed explicitly.
When the `disconnecting` flag was first added in
d44fdabea1, a client's
streams were not explicitly destroyed when it was freed,
instead, only the client's pw_core was disconnected,
which then caused all related streams to be destroyed.
Thus there needed to be a way to determine why the stream's
state changed to PW_STREAM_STATE_UNCONNECTED as it may have
been because the client is disconnecting or because the
stream has been "killed" in some other way.
The `COMMAND_{PLAYBACK,RECORD}_STREAM_KILLED` command
only needed to be sent to the client in the second case.
However, at the moment, all streams of a client are explicitly
destroyed before it's pw_core is disconnected. This makes the
`disconnecting` flag unnecessary, thus it can be removed.
Firstly, separate the message dropping logic into
its own `drop_from_out_queue()` function.
Secondly, do not check earlier messages if the NEW
event for a particular object has been reached while
processing a REMOVE event for that object.
Thirdly, if - while processing a REMOVE event -
the corresponding NEW event is found and dropped,
drop the REMOVE event as well.
See #1840
