Calls affected by metadata are constant

• Server: prevent feature implementers to change the calls affected by metadata, e.g. using this mechanism:
  • in Base class: add method set_calls_affected_by_metadata that should be used by Impl class, either in __init__() or start()
  • SilaServer.__start(): after starting all implementations, check if calls affected by metadata were set, otherwise raise an error
• Client: query affected calls once on connection and store the lists, users should not be able to execute the RPC
• Examples: Rework server and client example code to follow these guidelines