[qmf] fix read status sync error on large batches

JB#21962 [BUG][N.O.W] IMAP account on phone syncs unread statuses wrongly when large amount of messages change state

Updating thousands of read statuses leads to "too many SQL variables" error, similar fix was done in here: https://qt.gitorious.org/qt-labs/messagingframework/commit/4256ae33a7b5406d505968235f94d07f6510edf6