This change introduces tstime.NewClock and tstime.ClockOpts as a new way
to construct tstime.Clock. This is a subset of #8464 as a stepping stone
so that we can update our internal code to use the new API before making
the second round of changes.
Updates #8463
Change-Id: Ib26edb60e5355802aeca83ed60e4fdf806c90e27
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>