Add write-through local disk cache support

[mattgara]

It would be useful to have a write-through cache option where sccache can maintain both a local disk cache and remote cache simultaneously.

After initial build using remote cache, subsequent builds could benefit from local file access speeds and remove load off the network traffic.

Supporting this within sccache would be great, or interoperating with ccache is another option that works well too.

[trxcllnt]

sccache and ccache represent their caches differently, so sccache cannot use local ccache files.

Write-through caching for all cache types is in the latest release:

$ sccache --show-stats
<snip>
Cache location                  Local disk: "/home/ptaylor/.cache/sccache/"
                                s3, name: sccache, prefix: /sccache-dist-cache/
Use direct/preprocessor mode?   yes
Version (client)                0.12.0-rapids.5
<snip>

The new feature will write to/read from all configured cache types, e.g. if you've configured disk, S3, and GCS caches, it will use all three.

If an object isn't in the disk cache but is in S3, it will load from S3 and write it to disk. If a file isn't on disk or S3, but is in GCS, it will load from GCS and store it on disk and in S3. It will also write all objects to all caches (the write is considered a success if at least one of them succeeds).