API Changes:
basic_stream::expires_after used to take a nanosecond duration type argument. This
required users on systems where the steady_clock::duration_type was less accurate
to explicity duration_cast when calling this function, making code non-portable.
The duration type is now that of the embedded steady_clock.
Many thanks to @spdw for noticing this and providing a patch.
fixes#1943closes#1960