Fix tunnel stats not accounting for handshakes

[dlon]

The main difference I've seen compared to wireguard-go is that handshake initiations do not affect tx_bytes.

I've tried to make the behavior identical to that of wireguard-go. Any incoming packet that's accepted should contribute to rx_bytes, and any outbound packet on the UDP socket should contribute to tx_bytes. Except cookies. One difference is that wg-go adds MinMessageSize to incoming packet lengths (https://github.com/WireGuard/wireguard-go/blob/f333402bd9cbe0f3eeb02507bd14e23d7d639280/device/receive.go#L463).

Side-note: It might be more difficult than necessary to keep track of the side effects that different functions have on timers and counters.

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This change is 

[Serock3]

@Serock3 reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dlon).