HTTPSubprocessConnection: Run filter in a thread instead of process
HTTPSubprocessConnection: Run filter in a thread instead of process

This is a minor performance improvement in this use case (I
measured). Filter and HTTP processing can't really run in parallel,
because the latter needs the output of the former.

airtower-luna/mod_gnutls,airtower-luna/mod_gnutls,airtower-luna/mod_gnutls