Make snapshot directory configurable

Currently the snapshot directory is hard-coded to {table_dir}/snapshots/{tag}.

It should be possible to configure this to another directory, something like {snapshot_dir}/{table}/{snapshot_tag}.

Alternatively we could make a single directory for all snapshots making the global snapshot tag unique: {snapshot_dir}/{global_snapshot_tag}.

We should probably check that {snapshot_dir} is in the same disk as {storage_directory} to allow hard linking.