The previous implementation was using an unlimited TTL which would cause
problems in scale out deployments where multiple instances of the settings
service are running.
Fixes: #5067
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.
