Make transports pluggable

[gdamore]

The current transports are hard-wired into the library. We need to make these pluggable.

This will mean that transports will need to have access to the guts of the library, and we will need to change the transports fixed array into a linked list with registration routines.

It should not be necessary to support unregistering transports.

[gdamore]

It looks like the transports really only need access to a few nni_sock_xx calls and the aio framework. This is pretty clean.