1 - Centralized cache-awareness into Cache and pulled header details out of
SqliteCache and HttpClient
2 - Added support for expires header
3 - Support caching more status types (but not all, since HttpClient would need
to be aware of what caching a 3xx/206 means)
4 - Revalidate cares about "not specified" vs "no-store" vs "stale"
(e.g. expires=0 means "stale", not fallthrough the last-modified logic)
--http-cache-entry-limit is currently implemented as a hard-limit via a check
on every put. For a large --http-cache-entry-limit, this can result in a non-
trivial delay on every http get.
This commit calls cache.maintenance() on Browser.deinit and on startup.
maintenance:
1 - purges stale entries
2 - enforces the limit
With concurrent requests, the number of entries can easily exceed the limit by
hundreds of entries (but this is temporary).
Karl Seguin