Combine disk cache and gRPC cache

[borkaehw]

This is the second step of #8690. We should wait until #8740 is merged to master.

Fixes: #8690

[buchgr]

I am currently sending out some refactorings to get rid of SimpleBlobStoreActionCache completely and then allow GrpcRemoteCache extends SimpleBlobStore.

[borkaehw]

As long as GrpcRemoteCache becomes an implementation of SimpleBlobStore, it will solve #8740. Thanks.

[sitaktif]

Was this work abandoned? It's a pity, @borkaehw was so close...

[borkaehw]

Yeah, but Google decided to do it themselves. #10233 got us what we want, so we still get combined disk and gRPC cache. :)

[sitaktif]

Oh nice, I thought it wasn't there bacause disk cache is still incompatible with remote execution (which is well explained by the commit message):

      It does not yet also enable it for remote execution as
      this will need a bit more work. In particular it will
      require some refactorings around RemoteCacheClient#findMissingBlobs
      as when used with remote execution findMissingBlobs() should
      only check the remote cache and not the local disk cache. Else,
      it could happen that an input file is missing from the local
      cache but not from the remote cache in which case Bazel would
      attempt to upload the file again.

#8248 relates to disk cache + remote execution