Broadcast mem-cache writer departure to transient readers (in more/all cases).

Broadcast mem-cache writer departure to transient readers (in more/all cases).

Moved transientsAbandon() call to MemStore::disconnect() to make sure we
catch all cases where a mem-cache writer stops updating the cache entry.
Transient readers need to know so that they do not get stuck when a writer
disappears.

transientsAbandon() needs StoreEntry so MemStore::disconnect requires one now.