The Latest message data is stored in the database for each account.

Formerly, the message after that all offline messages were to be fetched
was determined by taking the latest message from the messages database
table.
If more recent messages were exchanged but then locally removed because
their corresponding chat was removed, the remaining latest message in the
database was not the latest exchanged message anymore.
That resulted in fetching the removed messages again instead of fetching
all messages after the most recent message of the removed ones.

The new approach avoids fetching messages of locally removed chats when the
exchanged messages were more recent than the latest one that remained
in the databse as well.
Thus, removed roster items are not added again by Kaidan's automatic roster
item addition on receiving the first message from an unknown JID when the
messages exchanged with that JID are more recent than the ones in the
messages database table after the local removal.

Closes #466