Improve client stream server side stub

Looking at this:
https://github.com/grpc/grpc-java/blob/master/examples/src/main/java/io/grpc/examples/routeguide/RouteGuideServer.java#L187

This is super weird to have the "request input", Points, as a return value of a method, and the response RouteSummary, to be the input parameter of a method. I took a look at this for a while to figure out what's going on. I see it is forcing the implementation of StreamObserver interface to process the incoming Points, and thus requiring it to be a return value. I wonder if there is a better way of doing this.

Every other language stubs seems to generate sensible stubs where the input steam is in fact the parameter and not a return value, and they actually returns the actual RouteSummary as the return value of the method.
https://github.com/grpc/grpc-go/blob/master/examples/route_guide/server/server.go#L102
https://github.com/grpc/grpc/blob/master/examples/node/route_guide/route_guide_server.js#L156
https://github.com/grpc/grpc/blob/master/examples/python/route_guide/route_guide_server.py#L93
https://github.com/grpc/grpc/blob/master/examples/ruby/route_guide/route_guide_server.rb#L144

Inizia con l’implementazione dello streaming lato server in examples/src/main/java/io/grpc/examples/routeguide/RouteGuideServer.java intorno alla riga 187, quindi confronta i server route guide in Go, Node, Python e Ruby collegati nell’issue. Determina se l’API dello stub Java può esporre il request stream e RouteSummary in modo più naturale; il lavoro è completo quando sono disponibili un design concordato e i corrispondenti aggiornamenti agli stub generati e all’esempio.

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