solver: rewrite to use Qspawn->psgi_qx and pi-httpd.async
The psgi_qx routine in the now-abandoned "repobrowse" branch
allows us to break down blob-solving at each process execution
point. It reuses the Qspawn facility for git-http-backend(1),
allowing us to limit parallel subprocesses independently of Perl
worker count.
This is actually a 2-3% slower a fully-synchronous execution;
but it is fair to other clients as it won't monopolize the server
for hundreds of milliseconds (or even seconds) at a time.