Reimplemented simple udp transport with one impl class.
Moved wait for ready/select implementation into common header.
Important note on select, timeval should have usecs < 1 second
or it may error on some platforms. Fixed in this implementation.
Simplified the fast path checking in the fw, but it turns out this was not the issue.
Fixed some bad bit operations with the 16sc words (dont forget sign extension).
Added some more documentation to the headers....
We can make simple udp transports for discovery and control.
We can support a udp zero copy transport (currently just asio).
Reworked the io_impl for usrp2 to work with the zero copy api.
So far, all of this untested other than compiling.
A cut-down vrt library is in the works to simplify the io impl.
