Failure to report Dropped CF without CF Handle destroyed before Close

[pdillinger]

If you drop a CF and later close the DB without destroying the CF handle, that can only be detected by a memory leak detector such as ASAN. I had thought this would be picked up by the ColumnFamilySet destructor at close time, but no it doesn't keep (weak) references to dropped column families.

This could mean users suffering significant, unexpected resource leak.