Introduce echo-message capability (3.3)

Suggest expanding to "A client may add an `id` tag to correlate between sent messages and echoed ones (which might have been modified by the server). If an `id` tag is present, the server must echo it."

&lt;can-of-worms&gt; Should the tag be forwarded or dropped? I'm 70% on the "drop" side; keep it simple for now. &lt;/&gt;