feat(net): add net2 crate and TcpBuilderConnector

[muja]

Use net2 crate that implements RFC#1158.

Fixes #602

[seanmonstar]

Ah bummer, I didn't notice that the net2 crate uses an unstable feature. This prevents me from merging this in hyper as-is. I could consider a possible cargo feature flag to include this, but the best would be to stabilize duration in libstd.

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[seanmonstar]

I'm going to close this, since the better #622 was merged.