Fix store_client caller memory leak on certain errors (#1347)

Fix store_client caller memory leak on certain errors (#1347)

When a storeUnregister() code path destroys store_client before the
latter has a chance to deliver the answer, the cbdataReferenceDone()
call in store_client::finishCallback() is not reached, keeping the
callback data (e.g., clientReplyContext) alive forever. These
storeClientCopy() "cancellations" may happen, for example, when the
client-to-Squid connection is closed while store_client waits for Store.

Use CallbackData to guarantee cbdataReferenceDone() when store_client is
destructed before it can finishCallback(). These synchronous callbacks
will be replaced with AsyncCalls. For now, we use the "discouraged"
CallbackData API to accommodate the existing legacy callbacks.

Probably broken since 2002 commit fa80a8e.