WT SizeStorer leaks memory

The WTSizeStorer is used to keep fast counts for each collection in the system. Creating a collection adds an `ident` -> `count` mapping, but those key/value pairs are never deleted.

RTT makes this problem worse. When rolling forward from the stable point -> common point re-creates a collection, it will get a new `ident`, thus leaking the previous ident in the table.