[RFC 7232] Conditional requests for cache entries revalidation

[krizhanovsky]

#515 requires sending conditional requests to origin server, so Tempesta must be able to generate and send If-Modified-Since requests and process corresponding responses from origin server. See RFC 7234 4.3.2 for the header processing logic in requests and 4.3.3 and 4.3.4 for appropriate response processing logic. The issues is a further development of #519.

There are 2 parts of the issue:

• send conditional requests to the origin server for requests for must-revalidate cached responses and conditional requests. It's much cheaper for the origin to reply with "not modified" than to build the response, probably with a database access.
• Send automatic revalidation (conditional requests) by the timer/eviction TDB thread to renew stalled cache entries. This once depends on TDBv0.2: Cache background revalidation and eviction #515 (TDBv2).

Tests

Please make a functional test for the feature as well.