better control of buffer sizes

[doudou]

As noticed by Janosch in #12, we're not controlling the receive buffer sizes. In the case of stream protocols (e.g. TCP), this is fine, but for e.g. UDP or serial, it is more problematic.

This is a proposal to add more control over these:

• have a Driver-level "get/setReceiveBufferSize()" attribute, expressed in in the number of packets it should hold. Default of 0 which indicates "no control".
• have a Driver-level "get/setInternalBufferSize()" attribute, expressed in the number of packets it should hold. Defaults to 2.
• let each I/O handler use the receive buffer hint or not.