Set END_STREAM flag when write_vio ntodo is 0

[masaori335]

Another approach to fix #6178.

[masaori335]

It looks like simple check of write_vio.ntodo() makes a problem with 100 Continue response for the POST request with expect: 100-continue header.

[masaori335]

The issue with 100 Continue response looks exists regardless of the dechunk cleanup changes. Filed as #6202.

[masaori335]

100 continue issue is addressed with new commit. I'll try applying the fix on the 8.0.x branch.

[shinrich]

These changes also correctly return a header only in the case of reponse-less statuses like 304.

[shinrich]

Will set up a performance build too.

[shinrich]

My test build on a prod box is looking good so far with respect to latency. Average latency staying around 120ms which is what I was seeing before the Dechunk changes. I'll build an official build tomorrow so I'm certain that I'm running the right thing. But so far, so good.

[masaori335]

Pushed new commits. Logics are almost the same as before.

• Now, this PR depends on Add 100-continue expectation support on H2 connection #6209.
• Add a test case (using "204 No Content" instead of "304 Not Modified" to make it easy)

[zwoop]

Oops, since there were two commits here, I missed one. I've cherry-picked both of these to the v9.0.x branch.