extindex: reduce IPC for cross-posted messages

extindex: reduce IPC for cross-posted messages

When dealing with cross-posted messages, we can reduce IPC
traffic by only sending the List-ID header(s) across the pipe
instead of all headers of a message.  List-ID headers only take
up a small portion of all message headers so the IPC traffic
reduction can be helpful in saving memory bandwidth to improve
performance.

This change will also make it easier to journal work for Xapian
and perform all work for cross posted messages in the same
transaction.  This ought to reduce doing updates for a given
document across transactions and hopefully reduce storage device
wear while improving performance.