[PATCH 2 of 4 RFC] chgcache: implement simple IPC mechanism

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-02-13 23:00:25 +0900:
> Why not os.pipe()?
>
> We share the same (dup-ed) file descriptors. In this case, the write end can
> be used by many forked processes, but IIRC the read end can't. Only one
> reader can read out a message. So I think there's no reason to set up a
> full duplex IPC channel.

Duplex IRC is not needed. But I do want to use DGRAM.

DGRAM makes it impossible to send incomplete messages. I think that makes
the code simpler and maybe more efficient.