Remote cache support

[mwichmann]

Creating a discussion to hold ideas about remote cache support.

This got started a few different places. There's an example implemenentation in PR #3971, and a proposal in issue #4394 (not the only places there have been disucssions, but these two have permanent links). Since these haven't progressed, we prefer to create a discussion page to capture the topics in the hope of making some progress, and leave links to earlier material here.

The main interesting question is whether the current CacheDir code is "abstract enough" to deal with different backends. It believes it's talking to a filesystem back-end. Any cached object has two interesting pieces: the data itself, and metadata that holds details about the data object. In a filesystem, you get the metadata by calling stat, and CacheDir does so directly, also directly interpreting the results. With remote storage you don't have the ability to call os.stat on the cached object directly.