Added is_disc_cache_active/0

[windkit]

No description provided.

[mocchira]

@windkit LGTM. just in case, please bench before/after applying this patch and using it on leo_gateway to confirm whether no performance degradation happens.

if non-negligible performance degradation happened then please consider using Props variable at https://github.com/leo-project/leofs/blob/develop/apps/leo_gateway/src/leo_gateway_s3_api.erl#L768 which contains enough information to judge if disk cache is active and passing those information through request_params that enables any http handler defined at leo_gateway_http_commons to refer those and no need to access ets at all.

[mocchira]

@windkit also please add .travis.yml for this repo.

[windkit]

@mocchira thanks for the comment, and now the leo-project/leofs#909 check the disk cache in http_options

[mocchira]

@windkit OK. I will check it.