This moves the ratelimit ('THUMBNAILS_MAX_CONCURRENT_REQUESTS') from the
HTTP endpoint to the GRPC endpoint. The HTTP endpoint is just used for
downloading already created thumbnails. But the resource consuming part of
thumbnail generation happens in the GRPC service.
TLS for the services can be configure by setting the "OCIS_HTTP_TLS_ENABLED",
"OCIS_HTTP_TLS_CERTIFICATE" and "OCIS_HTTP_TLS_KEY" environment variables.
Currently the ocis proxy is this only service that directly accesses backend
services. It determines whether to use TLS or not by looking a the new registry
metadata "use_tls". As specific CA Cert for certificate verification
can be set with the "PROXY_HTTPS_CACERT" environment variable.
