a first step for the duplicate message-id dilemma

[David Bremner]

These are mainly RFC because I'm not 100% sure about the performance
impact.  It seems OK for me: about 3% slower indexing my 500 K
messages with about 35k duplicates. I didn't see a noticable increase
in database size (both cases it's 5.8G / 3.5G before/after notmuch
compact).

There are also tons of UI issues: for example in the test case here,
notmuch search subject:'"message 2"' will happily print

thread:0000000000000001   2001-01-05 [1/1] Notmuch Test Suite; message 1 (inbox unread)

I claim it's still an improvement over the current code, where that
second message is not findable by any terms unique to it.