ipc: switch to one-way pipes

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.

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