To make it a bit easier to work in parallel we should split the NIA implementation into separate cpp files (similar to the replication coordinator).

Basic idea:

1) network_interface_asio.cpp

Contains implementation of all public methods.

2) network_interface_asio_command.cpp

all AsyncOp methods other than connect

3) network_interface_asio_connection.cpp

Contains AsyncOp::connect, and future methods for handling async DNS resolution and socket setup/teardown

4) network_interface_asio_ssl.cpp

Contains all aync SSL setup/teardown and integration with the SSL manager

5) network_interface_asio_auth.cpp

Contains async implementations of SASL, X509 and Mongo-CR. This may be split further in the future.