Support PackedForward mode to improve performance

[okkez]

Move from #73

• Fix ack handling #73 (comment)

@slang800 said:

It depends a lot on how you're doing packed forward mode. If you're trying to send messages as soon as possible then you're not going to be building up a backlog until you get well over 250 msg/sec (even if you limit to 1 outstanding ack at any given time), so messages will be sent individually up until that point. You could wait until you have a backlog of n messages before you pack them all together, but then if the process crashes up to n - 1 messages would be lost... And those messages would be from right before the crash, so they're probably important. You could pack and send a backlog of messages on an interval, but that creates the same type of problem n seconds of data could be lost in a crash.