[CBSE] Collect details of open snapshots

A stray open snapshot can cause all future snapshots that are closed to accumulate in memdb's gcsnapshots list. More information about this open snapshot can help identify why it is not being closed. This information for this one snapshot can be logged if the number of accumulated snapshots is very large. This can be calculated using the lastGCSn and currSn memdb stats.
 
Additionally, this information for all the open snapshots can be dumped at the time of cbcollect.