Return inserted-or-modified in store insert() result

[density215]

An insert() returns an integer, representing the amount of contention (the number of retries) that was encountered when inserting a prefix+metadata in a store, together with an Error.

But insert() is actually an upsert operation: it either creates a new (prefix, metadata) entry in the store, or it replaces the metadata on an existing entry with the metadata in the meta argument.

It would be useful to return what was actually happened in the store on insert, an insert of a new prefix or the modification of the metadata of the prefix.