Beginnings of a http cache

Doom cache entries based on the initial response, and prevent matching against doomed cache enties.

Evict cache entries that have passed their expiry date instead of matching them.

Document the cache. Refactor incomplete entries to lessen Option-itis.

Revalidate expired cache entries instead of unconditionally evicting them.

Forbid missing docs in cache code.

Revalidate must-revalidate entries.

Fetch content tests from a local HTTP server.

Track requests made to the test HTTP server.

Add a simple test that a cached resource with no expiry is not revalidated. Correct inverted expiry check in revalidation code.

Fix incorrect revalidation logic that dropped the consumer channels on the floor.

Ensure that requests are cached based on their request headers.

Run a separate http server instance for each test to avoid intermittent failures due to concurrent cache tests.

Add a test for uncacheable responses.

Address review comments.

components/net/lib.rs

@@ -48,6 +48,7 @@ mod data_loader;
 pub mod filemanager_thread;
 mod hosts;
 pub mod hsts;
+pub mod http_cache;
 pub mod http_loader;
 pub mod image_cache;
 pub mod mime_classifier;