Allow storing AC in different directory than CAS

[Yannic]

It would be great if bazel-remote had support for storing AC artifacts in a different directory (possibly different filesystem) than CAS artifacts. This would allow storing the small AC on fast SSDs and the large CAS on slower systems (e.g. normal hard disc or somewhere in the network), thus making build-without-bytes significantly faster. Background: I recently had to move my cache from the internal SSD to a (slower) external hard drive (because storage size), which added roughly 10 minutes to my builds.

[mostynb]

This is already possible with bazel-remote's existing cache directory structure, if you mount the filesystems in the way that you want before starting bazel-remote.

However there is no separate cache size accounting for AC and CAS, which means you might run out of storage on one of the filesystems (and then bad things would probably happen). You might be able to run some benchmarks though, to see if this would be effective if we could figure out a way to manage the storage size.