PROTON-2131: Introduce new proton logging API

[astitcher]

See PROTON-2131 for some of the problems this API seeks to solve.

This change

• Introduces a new type pn_logger_t which represents a logger

  • The library has a default logger this can be retrieved using pn_default_logger()
    All new loggers will inherit their properties from the default logger at the point they are created. The default logger will also be used for any messages logged outside the context of an AMQP connection.
  • In the current proton code every pn_transport_t object also has a logger, which will be initialised from the default logger when the transport is created. All messages logged in the context of an AMQP connection will be logged to the connection's transport's logger.
    The transport's logger can be retreived using pn_transport_logger().

• Converted all uses in the Proton core library to use the new API

• Currently no higher language API bindings (Python/Ruby/C++)

• This introduces some portable library infrastructure to globally initialise/
  shutdown the library independent of it's clients.