Refactor ServiceDescriptorsUpdater out of CachingServiceSelector
Use ServiceDescriptorsUpdater to populate HttpServiceBalancerImpl
Remove support for loading service inventory over http/https.
Create a discovery HttpServiceBalancer from ServiceInventory
Use DiscoveryAsyncHttpClient for talking to discovery service
Pass HttpServiceBalancer exceptions to ResponseHandler
Make ServiceUnavailableException not depend on type/pool.
Move most of BalancingHttpClient to http-client module