Design fair servicing for sustained VM and host traffic

[edi-oai]

See #196 (comment).

We currently have an exception for service_control(), which runs each 128 packets:

softnet/lib/proxy/mod.rs

Lines 160 to 163 in df0874b

	if packets_read == 128 {
	self.service_control();
	packets_read = 0;
	}

It would be nice to assess the security and performance implications of either:

1. not doing this; or
2. doing this for other periodic routines like port_forwarder.tick() too

And implement a more consistent approach.