Compute checksum if crc32c tlv is send in proxy-protocol-v2 header.

Parse and check tlv and ssl sub tlv as described in:
https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

If the stevedore failed the object creation, we would leak the temporary
VSB holding the computed vary string. This patch frees it.

Problem exists in 4.1 and later.

I had hoped for a more explanatory error message, but that is too
involved at this point.

Properly get rid of streams that failed to reschedule after being
waitlisted.

Fixes: #2563
Fixes: #2592