MEDIUM: tree-wide: replace timeval with nanoseconds in tv_accept and tv_request

MEDIUM: tree-wide: replace timeval with nanoseconds in tv_accept and tv_request

Let's get rid of timeval in storage of internal timestamps so that they
are no longer mistaken for wall clock time. These were exclusively used
subtracted from each other or to/from "now" after being converted to ns,
so this patch removes the tv_to_ns() conversion to use them natively. Two
occurrences of tv_isge() were turned to a regular wrapping subtract.