Ensure StoreEntry presence during store_client lifetime (#1281)

Ensure StoreEntry presence during store_client lifetime (#1281)

Most likely, all store_client users already lock their StoreEntry
objects, so this change is unlikely to affect any current use cases.
However, objects like store_client that store StoreEntry objects for
long-term/asynchronous reuse should lock their entries rather than rely
on others to do that for them. Being able to rely on StoreEntry locking
also clarifies existing logic in several store_client methods.