Remove serial signalling protocol from producers to consumers

[Courtney3141]

Currently, both the consumer and producer of a serial queue only signal each other if their corresponding flags are set - there are two flags for each queue. This is in contrast to the networking subsystem, where the consumer never signals the producer, and the producer only signals the consumer depending on the value of the flag.

Since the serial subsystem is using a signalling protocol for both directions of communication, it makes the code quite complicated to read and understand for those who are unfamiliar with the protocol. Since the serial subsystem is not performance critical, and the risk of over signalling even with unconditional signalling is very low, I have decided to remove one direction of the signalling protocol so that producers of serial queues signal consumers unconditionally after a batch of work has been enqueued.

The consumer side of the protocol remains unchanged, as typically producers do not require signals from consumers - it is only required when a queue has become full and there is still more data to be enqueued. Since under normal conditions this does not occur frequently, it makes sense to keep the protocol for this direction of communication.

Merge after #301.

The base branch was changed.