Removed StoreClient::created() and improved PURGE code quality (#734)

Removed StoreClient::created() and improved PURGE code quality (#734)

The StoreClient::created() callback method was probably added in hope to
make Store lookups asynchronous, but that functionality has not been
implemented, leaving convoluted and dangerous synchronous created() call
chains behind. Moreover, properly supporting asynchronous Store lookups
in modern code is likely to require a very different API.

Removal of created() allowed to greatly simplify PURGE processing,
eliminating some tricky state, such as `purging` and `lookingforstore`.

Also removed all Store::getPublic*() methods as no longer used.