Add packet flushing logic to acknowledgement flow

[crodriguezvega]

Pre-requisite:

• ics04: Channel Upgrade Packet Flushing Logic ibc#982

After deleting the packet commitment, change flush status to FLUSHCOMPLETE if the flush status is FLUSHING and there are no pending in-flight packets.

Question: to follow a similar pattern as we do for ChanOpen[Try, Ack, Confirm] message server handler functions where we have functions WriteOpen[Try, Ack, Confirm]Channel writing the channel state, should we have also a WriteAcknowledgePacketChannel (or with different name) function that writes the change of channel state? Since the logic is the same also for TimeoutPacket and TimeoutOnClose maybe we can have one implementation that can be used for all three.