When sniffing detects TLS but the ClientHello has no SNI, HandleTLS
errors with 'tls: sni is empty, closing connection'. Previously this
error was returned unconditionally even with sniffingFallback enabled.
Now, bytes consumed by the sniffer are captured via io.TeeReader and
replayed through io.MultiReader when sniffingFallback is true, allowing
the connection to fall through to raw forwarding to the original dstAddr.