refactor(handler/http2): split handler into 3 focused files with 52 tests

Extract authenticate/probe-resistance to auth.go and roundTrip/forwarding
to proxy.go, following the handler/http/ pattern. Export sentinel errors
(ErrAuthFailed, ErrWrongConnType) for precise test assertions. Fix
flushWriter panic recovery to handle non-string/non-error panics. Default
HTTPS port to 443 when no port is specified in the Host header.

handler/http2/metadata.go

@@ -25,7 +25,7 @@ type metadata struct {
 	limiterCleanupInterval time.Duration
 }
 
-func (h *http2Handler) parseMetadata(md mdata.Metadata) error {
+func (h *http2Handler) parseMetadata(md mdata.Metadata) {
 	if m := mdutil.GetStringMapString(md, "http.header", "header"); len(m) > 0 {
 		hd := http.Header{}
 		for k, v := range m {
@@ -57,8 +57,6 @@ func (h *http2Handler) parseMetadata(md mdata.Metadata) error {
 
 	h.md.limiterRefreshInterval = mdutil.GetDuration(md, "limiter.refreshInterval")
 	h.md.limiterCleanupInterval = mdutil.GetDuration(md, "limiter.cleanupInterval")
-
-	return nil
 }
 
 type probeResistance struct {