Protect against monotonic time being non-monotonic.
Protect against monotonic time being non-monotonic.

Within utp.cpp, there's a bunch of assertions that will cause us
to crash if monotonic time isn't.  While we're in principle already
protecting in the cases where non-monotonicity is likely (see the
gettimeofday case for generic Unix, for example), this protects for
all versions.

bittorrent/libutp,wujf/libutp,anacrolix/go-libutp,robgjansen/libutp,robgjansen/libutp,bittorrent/libutp,bittorrent/libutp,wujf/libutp,anacrolix/go-libutp,anacrolix/go-libutp,anacrolix/go-libutp,robgjansen/libutp,wujf/libutp