Sub-millisecond [autovacuum_]vacuum_cost_delay broken

Hi,

I think that 4753ef37e0ed undid the work caf626b2c did to support
sub-millisecond delays for vacuum and autovacuum.

After 4753ef37e0ed, vacuum_delay_point()'s local variable msec is a
double which, after being passed to WaitLatch() as timeout, which is a
long, ends up being 0, so we don't end up waiting AFAICT.

When I set [autovacuum_]vacuum_delay_point to 0.5, SHOW will report that
it is 500us, but WaitLatch() is still getting 0 as timeout.

- Melanie