httpd: psgix.io is the underlying IO object

httpd: psgix.io is the underlying IO object

PSGI::Extensions(3pm) states psgix.io should be "the raw IO
socket", so we shall conform to that specification.  For our
codebase to continue taking advantage of
public-inbox-daemon-specific features, we now expose the
PublicInbox::HTTP object via $env->{'pi-httpd.client'}.  The
`pi-httpd.async' boolean field is now gone, since we can just
test for the presence of `pi-httpd.app' or `pi-httpd.client'
to avoid occupying another hash table entry.