Consider using streaming responses for rpc requests with potentially large output

[bigspider]

GRPC allows streaming responses; perhaps that would be more appropriate for requests like get_all_appointments that might have a very large output.

We should also make sure that the read lock is released quickly even if the response is large; if needed we could make a copy of the returned data and returning that copy after releasing the lock (since those requests come from the rpc − therefore from the cli − DoS attacks are not a concern).