Fix ENTRY_ABORTED assertion in sendClientOldEntry()

[rousskov]

FATAL: assertion failed: client_side_reply.cc:392:
"!EBIT_TEST(http->storeEntry()->flags, ENTRY_ABORTED)"

The replaced assertion was wrong because a stale entry may be aborted
while we are revalidating it. The exact real-world conditions that
triggered this assertion are unknown, but many conditions lead to
aborted entries. The assertion can be triggered in lab tests using a hit
transaction that collapses on a miss transaction that runs into storage
problems (e.g., a memory cache that ran out of usable space).

Also adjusted cacheHit() to avoid similar problems. We have not
reproduced them, but no code maintains the asserted invariant on the
cacheHit() path either. Moreover, async-called cacheHit() initiates
processExpired() that leads to problematic sendClientOldEntry() call, so
seeing an aborted StoreEntry at cacheHit() time is probably a matter of
sufficient concurrency levels and asynchronous callback delays.

[rousskov]

FWIW, I have checked other ENTRY_ABORTED assertions in src/client_side_reply.cc and did not find any additional ones that were similar enough to the two removed ones. Some of the remaining assertions are questionable, but their removal (if they should be removed) requires more work and somewhat different justification.

[kinkie]

LGTM