Share an http cache between build workers

[aleclarson]

Repeating HTTP requests leads to slower builds, so workers should share them.

[aleclarson]

We can use atomically to safely write to the .saus/http-cache/metadata.json file between processes. Then read the metadata on every .read call.

If we can also track pending requests somehow, avoiding the duplication would be nice.

[aleclarson]

This is high priority for Parallel Rendering to be fully operational.