Bug 3480: StoreEntry::kickProducer() segfaults in store_client::copy() context

Bug 3480: StoreEntry::kickProducer() segfaults in store_client::copy() context

Short-term fix: Lock StoreEntry object so that it is not freed by
storeClientCopy2() callbacks. Also lock StoreEntry in storeUnregister()
context because an aborting entry may be deleted there unless it is
double-locked.

See bug 3480 comment #27 for detailed call stack analysis. Additional cases
include rejected copied HIT due to Var mismatch and hits blocked by
reply_from_cache directive (under development; see bug 3937).

Long-term, we need to make store copying asynchronous and revise StoreEntry
locking approach.