move SSE-C TLS enforcement into generic handler (#6639)

This commit moves the check that SSE-C requests
must be made over TLS into a generic HTTP handler.

Since the HTTP server uses custom TCP connection handling
it is not possible to use `http.Request.TLS` to check
for TLS connections. So using `globalIsSSL` is the only
option to detect whether the request is made over TLS.
By extracting this check into a separate handler it's possible
to refactor other parts of the SSE handling code further.

cmd/routers.go

@@ -82,6 +82,8 @@ var globalHandlers = []HandlerFunc{
 	// routes them accordingly. Client receives a HTTP error for
 	// invalid/unsupported signatures.
 	setAuthHandler,
+	// Enforce rules specific for TLS requests
+	setSSETLSHandler,
 	// filters HTTP headers which are treated as metadata and are reserved
 	// for internal use only.
 	filterReservedMetadata,