Threading: prevent dangling persistent indexes when removing multiple messages

Commit faea6a3132fe78fd25e84e7adeeb3edddb89b46b introduced a regression -- when
multiple messages were selected, marked as deleted and subsequently expunged,
the selection reappeared and included new messages which were now at the same
(or similar?) place. This patch fixes one corner case which might have led to
this behavior.

REVIEW: 110705
Merge branch 'threading-persistent-indexes'
abook: don't offer unrecognized addresses

The perfect fix would be, of course, to make the MailAddress::fromPrettyString
support even "foo@example.org <foo@example.org>", but that look like a lot of
work (and besides that, I don't want to see these regexps in the next month or
so)

BUG: 320499
REVIEW: 110740
Composer: support address completion even on chunks starting with ".", "_" and "-"

REVIEW: 110738