The driver needs to be able to manage the application's non-AMQP file descriptors

A common integration use-case is for Proton to be integrated into an application that uses sockets or file-descriptors for purposes other than AMQP communication. There are two general cases here: The program already has a main loop based on select/poll/epoll; or the program wishes to use the Proton driver's main loop to manage its user fds.

In the first case, the integrator is likely to write his own driver. In the latter case, it would be useful if the driver has the ability to manage a "foreign" file descriptor in its own poll loop. This Jira is only about the latter case.

This can be accomplished by using pn_connector_fd() to create a connector for an external fd. This connector will never be "processed" as it does not represent an AMQP connection. By adding activation and is_activated hooks, the application can then allow the Proton Driver to manage all its fds.