Make objects read-only, once they have been stored.

Now that we have a proper purge-mechanism, all the magic to
set obj.{ttl,grace,keep} in vcl_hit{} can go away.
Move retries behind refcount so it is cleared when busyobj is recycled.

Spotted by:	martin
If vcl_synth{} or vcl_deliver{} sets a "Connection: close" header,
we close from our side with the new SC_RESP_CLOSE status.
Add tests for outgoing Connection: close handling
Reflect change of obj size
Bump 10 bytes further to catch all platforms
more tweaking
more tweaking
Add a couple of tiny delays to stabilize this test.
Stabilize this test with a short sleep
Handle 'return (abandon);' in vcl_backend_response.

Test case by: Federico G. Schwindt

Fixes: #1417
Add more panic detail on busyobjs.

Remove a debugging message.