[MERGE] Move RemoteRepository caching logic to CachedParentsProvider

[Andrew Bennetts]

Aaron Bentley wrote:
> Hi all,
> 
> This patch reduces code duplication and api friction, and increases
> functionality.
> 
> It moves the core cache management logic of RemoteRepository into
> CachedParentsProvider, but retains CachedParentsProvider's ability to
> cache missing entries as well as found entries.

bb:resubmit

This patch breaks several RemoteBranchForamt tests in
branch_implementations.test_push and .test_pull, and similarly many
per_repository tests too.

I think this is because it unfortunately isn't safe to cache misses in
RemoteRepository, because the cache of misses doesn't get invalidated when
revisions are added to its _real_repository.

-Andrew.