Stale cached responses and immediate purging

[krizhanovsky]

Need to test new logic for https://github.com/tempesta-tech/tempesta/issues/522 (see also the implementation in https://github.com/tempesta-tech/tempesta/pull/2074 ):

• [ ] some stress test, which causes web cache eviction (e.g. DDoS scenario with random URLs). Ideally, if you can also test clients eviction
• [x] cache_use_stale directive and the described logic
• [ ] client_lru_size directive
• [x] handling of the stale-if-error Cache-Control header extension from RFC 5861: caching logic, parsing, including doubled values and incorrect values
• [x] immediate purging