The latest version of the BIP doesn't use inversion, but does use
bitshifts.
It also uncovered a bug in the test scripts: the block timestamps
creep forward when we generate large numbers of blocks (UpdateTime
insists it be > GetMedianTimePast() so it's valid). We need to take
this into account when waiting for the median to move (reduced it from
60 to 30 seconds, since that adds about 14 seconds).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
