This change adds a new streaming response to the FindManifests API. The
server will deliver the response in chunks of N manifests where N is
requested by the client. This allows the client to process the response
in chunks and improves pipelining of responses.
For now client will hold the entire response in memory since this
is what FindManifests() API currently does. This will be fixed in a
follow up change.
Replaces #2713Fixes#2660