e2e: cover egress for non-HTTP traffic, in both speaking orders

[haiyanmeng]

This PR adds e2e test coverage for non-HTTP traffic, in both speaking orders. It adds the following:

• /tcp on the egress demo opens a raw TCP connection and reads before it writes,
  so an empty banner really does mean the peer stayed silent rather than that the
  probe got the ordering wrong.
• bannerserver is an in-cluster TCP origin that greets on accept and then echoes.
• bannerserver is an in-cluster TCP origin that echoes on two ports:
  on one it greets on accept, on the other it stays silent until
  spoken to. Two ports, because the server has to decide whether to
  greet before it has read anything, so nothing in the request could
  select the behavior -- only the address dialed can.
• TestActorEgressRawTCP dials both ports through one Actor and
  requires, per port, both the CONNECT record and the byte counters on
  the gateway's close-time access log, which is what shows the gateway
  relayed the payload rather than the Actor having reached the origin
  some other way.
• TestActorEgressSSH is the same probe against github.com:22.

Addresses #1017

It's a good idea to open an issue first for discussion.

• Tests pass

[Bowei Du (bowei)]

This is great, do we also need "client speaks first" non-HTTP protocol test case (do we have that already?).
You could do it in the same test -- have server bind two ports, one where client is sending request first, the other port where server sends first...

[haiyanmeng]

do we also need "client speaks first" non-HTTP protocol test case (do we have that already?).
You could do it in the same test -- have server bind two ports, one where client is sending request first, the other port where server sends first...

Done