Resource cleanup in cnt_error() when Transient is full.

Fixes: #1283
Properly clean up resources after STV_NewObject fails before jumping to error.

Fixes: #1284
Be able to detect parse error during processing of backend Vary
headers, and do 503 on parse error.
Remove dodgy workaround introduced for #763.

With fix for #1275 we can now do a proper 503 when malformed
Vary-headers are received from backend. Error out instead of accepting
bogus ':'.
Don't panic on malformed Vary headers.

Fixes: #1275

Test case by: Dag Haavi Finstad
Return 503 when Vary-headers references header names more than 127
(out limit) characters long.

Fixes: #1274

Test case by: Dag Haavi Finstad
Throw 503 on exceeding max 65k Vary header constituent limit.
Document Vary parse error behavior