Allow setting QoS parameters like prefetch size and count

Setting `qos_prefetch_size` will define maximum amount of content (measured in octets) that the server will deliver
(0 if unlimited - default).

Setting `qos_prefetch_count` will define maximum number of messages that the server will deliver (0 if unlimited).
Default to `bulk_size*2`.

Closes #38.
Closes #55.