[ContBk] The user should be alerted when there is a hole in the backup

In continuous backup it is possible for there to be a hole in the logs file for a few different reasons:

• a failover happened and we did not start backing up on the new active until longer than the history retention window
• hardlinking failed and by the time it was retried the list of files to hardlink was too large to keep in memory
• writing the metadata file out failed

We should alert the user when a hole happens, as to remedy it they will need to take a traditional backup. cbcontbk will expose a metric called cont_bk_holes which will be incremented whenever one is found