Add new_server_side_from_acceptor to allow init from a pre-existing Acceptor

[mmastrac]

In certain cases, a TLS server may need to pre-process the incoming bytes (eg: it may want to peek to see if this is raw HTTP, or HTTP over SSL) before deciding to hand it off to a TlsStream. Because TlsStream uses raw TcpSocket, there's no way to re-inject any sniffed bytes that were read() rather than peek()'d from the socket.

This adds a new new_server_side_from_acceptor where an initial Acceptor can be passed in. It may contain either the full handshake or just a few prefix bytes that were sniffed from the socket earlier.

We test this by creating a new TLS pair where the socket sniffs the first 8 bytes of the client handshake before firing up its TlsStream.

[bartlomieju]

LGTM!