conntrack: add connection tracker wrapper based on connfu

[mmatczuk]

Use builder pattern to return implementation with only required features. Use connfu to handle CloseWrite() and ReadFrom() propagation.

[Choraden]

Although tracked seems cool, we could use some fancy name to emphasise it's a real net.Conn on steroids.
Some proposals:

• connx
• turboconn
• hyperconn
• connboost

[mmatczuk]

Traditionally the name is conntrac not to get confused with conntrac in Linux.

[mmatczuk]

Renamed package to conntrack, added Build() and BuildWithObserver() to simplify usage.

[Choraden]

@mmatczuk This looks good!

1. I wonder if we want to add assertion in martian that the splice optimisation is to take place.
2. I would love to see some numbers on how it improves performance.