Bug 559391 - Wrong number of unread mails in folder tree

This reverts part of the original fix for this bug which stopped reading
from the summary database and caused all kinds crazy message counts to
appear in Evolution's folder tree (often negative counts, or negative
counts cast to an unsigned integer).

Message counts are back to normal now.

Reverts part of commit 42489b26.

(cherry picked from commit 1422d8fc82b8c2d11b782ac21c5eb57e9fc1042c)