treat message update and delete differently if message is invocation of currently processing command

[Jacherr]

There is a special case in the gateway handling logic, which involves the case as follows:

• A user invokes a command, in particular a slow one.
• The command begins processing, and during the processing phase, i.e., while the command function is executing, the invocation message is updated or deleted.

This needs to be handled differently from usual message updates or deletes. In particular:

• If the invocation is updated to a new command, ignore it, to avoid overwriting the currently executing command,
• If the invocation is updated to a non-command message, stop responding to the invocation and delete any existing response, to avoid a 'ghost command', i.e., a command with no apparent invocation,
• If the invocation is deleted, do the same as the second point.

This will need to be handled by tracking message events, and seeing if a currently cached Reply object has its invocation message_id updated or deleted. If so, check the status of that command with ReplyState to see if it still ReplyState::Processing. If so, perform one of the above actions.