Fetcher should not depend on SlaveID.

Currently, various Fetcher interfaces depends on SlaveID, which is an unnecessary coupling. For instance:

Try<Nothing> Fetcher::recover(const SlaveID& slaveId, const Flags& flags);

Future<Nothing> Fetcher::fetch(
    const ContainerID& containerId,
    const CommandInfo& commandInfo,
    const string& sandboxDirectory,
    const Option<string>& user,
    const SlaveID& slaveId,
    const Flags& flags);

Looks like the only reason we need a SlaveID is because we need to calculate the fetcher cache directory based on that. We should calculate the fetcher cache directory in the caller and pass that directory to Fetcher.