This fixes a performance regression in multi-process v2 indexing
due to the switch to PublicInbox::IPC. While Unix sockets are
fewer FDs to manage, pipes allow unprivileged processes to use
larger buffers (up to 1M) on out-of-the-box Linux instances.
A larger buffer via F_SETPIPE_SZ afforded by pipes was proven
valuable during v2 development in 2018 and continues to be
valuable when we get significant amounts of one-way traffic from
the producer parent to worker children.
Compression may be an option for systems without F_SETPIPE_SZ;
but it increases CPU usage with no memory bandwidth savings on
hosts where larger buffers are available.