Robotics Service Bus

Avoid unsafe use of asio::io_service in src/rsb/transport/yarp/Connector*.cpp

fixes #1223

The problem was that the Nameserver class, when used by the
OutConnector class, could get an asio::io_service without a
thread. This would cause asio to spawn a thread which in turn could
outlive some of the objects it accessed.

The solution consists in adding an ExecutionContext class which
manages the asio::io_service and its thread and is used by the
*Connector and Nameserver classes.

• src/rsb/transport/yarp/Execution.{h,cpp}: new files; contain
  ExecutionContext and ContextProxy classes
• src/rsb/transport/yarp/ConnectorBase.{h,cpp}: new files; contain
  ConnectorBase class which handles access to the ExecutionContext,
  holds the asio socket and performs basic state management
• src/rsb/transport/yarp/Nameserver.{h,cpp}: use a ContextProxy to
  access the asio::io_service
• src/rsb/transport/yarp/InPushConnector.{h,cpp}: added ConnectorBase
  base class; used for state management, socket and scope store and
  access to the asio::io_service
• src/rsb/transport/yarp/OutConnector.{h,cpp}: similar
• src/CMakeLists.txt: added files
  src/rsb/transport/yarp/Execution.{h,cpp} and
  src/rsb/transport/yarp/ConnectorBase.{h,cpp}