settings: Instanciate only a single instance of the ServiceHandler/Store

Share the same instance between the HTTP and the GRPC service. This is
in preparation for moving the cache of the metadata storage backend to a
go-micro/store based implementation. By sharing the same service instance in
the HTTP and GRPC services we can avoid the usage of global variables for the
caches, which will make the move to the go-micro/store implementation simpler.
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