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

Stream threads may signal a cleanup sweep by setting h2->do_sweep

This removes the need for non-VRT VMODs to require a Varnish source tree
to build, especially since varnish.m4 only supports Varnish installations
today and would leave VMOD authors on their own in order to build from
a source tree. Since $ABI defaults to strict, any VMOD author actively
choosing vrt compliance is responsible for ensuring it only uses blessed
symbols.

In our official packages, they land in the varnish-dev deb and the
varnish-devel RPM:

    $ rpm --query --package --list varnish-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h

    $ rpm --query --package --list varnish-devel-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h
    /usr/include/varnish/cache/cache_varnishd.h

You will have to trust me for the Debs as it's a bit cumbersome for me to
build them locally.

Not that I'm planning another stab, but please review and let me know if
anything needs clarification.

Currently not backends.