client: add a bunch of comments and function docs
device: more aggresively send ACKs when receiving TCP data

Before this it seemed that we only ever sent ACK when
timing out... Looks like a perf double when reading.
device: fix potential integer overflow in mstime64() on 32 bit systems
device/utils: move mstime64() into utils since it is generally useful
usb-linux: add some explanatory comments to usb-linux.c
usb-linux: massive read perf improvement with 3 parallel transfers

By maintaining 3 parallel usb trasfers when reading we get 2-3x more
throughput when reading. Without this the usb port is mostly just idling.
I get 23mb/s on my system compared to a clean Apple stack that gives me
17mb/s.

3 was chosen because it is simple to hard code, gives very good performance,
and have very little impact on out resource consumption.