Use a single unified pass to fetch scalar message metadata.

Use a single unified pass to fetch scalar message metadata.

This performs a single pass over a message's term list to fetch the
thread ID, message ID, and reply-to, rather than requiring a pass for
each.  Xapian decompresses the term list anew for each iteration, so
this reduces the amount of time spent decompressing message metadata.

This reduces my inbox search from 3.102 seconds to 2.555 seconds (1.2X
faster).