Bug 3346: forward.cc:364: "entry->store_status == STORE_PENDING"

Bug 3346: forward.cc:364: "entry->store_status == STORE_PENDING"

While the server writes the response to Store, the client side may
synchronously abort the entry. This happens, for example, when the
server receives a 304 response and handleIMSReply calls
sendClientOldEntry, which calls storeUnregister with our entry,
resulting in CheckQuickAbort.

Once server store write returns, if the server is done, it calls
FwdState::completed(). At that time, the server does not know that (and
should not care whether) the entry was aborted. Thus, we need to handle
aborted entries in FwdState::completed.

This is a Measurement Factory project.