The FS message store truncates message times to the second.
This means that a message sent out as 2020-01-01T00:00:00.123Z could be
sent later as part of a CHATHISTORY batch as 2020-01-01T00:00:00.000Z,
which could cause issues in clients.
One such issue is a client sending a MARKREAD for
2020-01-01T00:00:00.000Z, with another client considering the
2020-01-01T00:00:00.123Z message it has as unread.
This fixes the issue by truncating all message times to the second when
using the FS message store.
