Include TLS overhead bytes into connection stats for HTTPS upstreams

[bliuchak]

Legacy issue #607

TLS overhead bytes are missing from final connection stats between HTTP/HTTPS proxy (proxy-chain) and HTTPS upstream.

[bliuchak]

Giving this low priority by default.

Based on current state of HTTPS upstreams in proxy-chain (lack of tests, missing TLS bytes), suggesting to put this on-hold till there will be clear demand.