Search for multiple related messages using In-Reply-To and References
RFC822 In-Reply-To and References headers are both lists of message ids, but Geary currently treats In-Reply-To as a single id (it usually is) and References is not consulted at all.
This means conversations may miss out on some related messages, and also forwarded messages are not always placed in a conversation.
LIKE clause for the two headers requires
COLLATE NOCASE indexes (since
LIKE is case-insensitive), but placing a wildcard
% at the start of the string prevents the index being used. Using a function like
instr (per commit 852e8144) won't every use an index.
So we either set up FTS tables for these, split them out into their own individual association tables, or… something else?