FETCH BODY[section]<partial> support.

This adds support for retrieving partial header and body blocks straight from the email, and
therefore support to pull the References header from a message (which, for some reason, IMAP
doesn't support or include in the FETCH ENVELOPE command).  This is necessary for email conversations (#3808).

This required a change to the database schema, meaning old databases will need to be blown
away before starting.

sql/Create.sql

@@ -33,6 +33,7 @@ CREATE TABLE MessageTable (
     
     message_id TEXT,
     in_reply_to TEXT,
+    reference_ids TEXT,
     
     subject TEXT,