feat: introduce TLSConn abstraction and use it consistently

[bassosimone]

With this abstraction in place, it's quite easy to use
github.com/refraction-networkings/utls with this net/http fork.

> git grep 'tls\.Conn'
cgi/child.go:           r.TLS = &tls.ConnectionState{HandshakeComplete: true}
h2_bundle.go:   ConnectionState() tls.ConnectionState
h2_bundle.go:// client. If c has a ConnectionState method like a *tls.Conn, the
h2_bundle.go:           sc.tlsState = new(tls.ConnectionState)
h2_bundle.go:   tlsState         *tls.ConnectionState        // shared by all handlers, like net/http
h2_bundle.go:                   // TODO: add CloseWrite to crypto/tls.Conn first
h2_bundle.go:   var tlsState *tls.ConnectionState // nil if not scheme https
h2_bundle.go:   // If the returned net.Conn has a ConnectionState method like tls.Conn,
h2_bundle.go:   tlsState  *tls.ConnectionState // nil only for specialized impls
httptest/httptest.go:           req.TLS = &tls.ConnectionState{
httptest/httptest_test.go:                              TLS: &tls.ConnectionState{
httptrace/trace.go:     TLSHandshakeDone func(tls.ConnectionState, error)
request.go:     TLS *tls.ConnectionState
response.go:    TLS *tls.ConnectionState
server.go:      tlsState *tls.ConnectionState
server.go:              c.tlsState = new(tls.ConnectionState)
server.go:              req.TLS = &tls.ConnectionState{}
tlsconn.go:// TLSConn is the interface representing a *tls.Conn compatible
tlsconn.go:// connection, which could possibly be different from a *tls.Conn
tlsconn.go:     ConnectionState() tls.ConnectionState
transport.go:                   trace.TLSHandshakeDone(tls.ConnectionState{}, err)
transport.go:                                   trace.TLSHandshakeDone(tls.ConnectionState{}, err)
transport.go:   tlsState  *tls.ConnectionState
transport_test.go:              TLSHandshakeDone: func(s tls.ConnectionState, err error) {
transport_test.go:              TLSHandshakeDone: func(s tls.ConnectionState, err error) {
transport_test.go:                      TLSHandshakeDone: func(cfg tls.ConnectionState, err error) {