when clustered, there are chances the a mqtt client process
get killed (e.g. holding the channel registeration lock for too long),
if the channel data inserts happen before casting out the message
for channel process monitoring, there is a chance for the
stale message left in the ets tables indefinitely.
this commit changes the order of the non-atomic operations:
it casts out the monitor request message before inserting
channel data.
```
%%% emqx_channel_SUITE ==> t_handle_kicked_publish_will_msg: FAILED
%%% emqx_channel_SUITE ==> will_message_not_published
```
The problem was that the whole generated message was compared for
equality when receiving the will message, but the timestamp may be
different at publishing time...
