Backup cursor open should force a log file switch

Right now when logging is enabled and we open a backup cursor, we get the list of log files to copy, and, as part of doing that, force out all earlier writes.

I believe that WiredTiger needs to also force a log file switch so that additional and later operations that occur are not visible to the backup and would be replayed and recovered when the backup is opened. The log should also represent the point in time when the backup was opened.