EST_INTRO_DOS_EXT: Tighten up (and clarify) invalid parameter cases
Fixes #167.
This answers the questions there along the lines of @dgoulet's answer #167 (comment 2828245)
Additionally, we specify the ordering (as a SHOULD for the sender), and specify to reject burst < rate
(rather than ignoring such invalid parameters as the spec says now).
I have not looked at the code in C Tor to see what it does. If this MR is accepted, probably both C Tor and Arti should be checked for conformance.