Avoiding roundoff error in pg_sleep()

I chanced to notice that if you ask pg_sleep for 1ms delay,
what you actually get is 2ms, for example

regression=# \timing on
Timing is on.
regression=# do $$ begin
for i in 1..1000 loop
perform pg_sleep(0.001);
end loop; end $$;
DO
Time: 2081.175 ms (00:02.081)
regression=# do $$ begin
for i in 1..1000 loop
perform pg_sleep(0.002);
end loop; end $$;
DO
Time: 2177.407 ms (00:02.177)

A bit of gdb-ing confirms that the delay passed to WaitLatch() is 2ms,
so the problem is fundamentally one of floating-point roundoff error
in pg_sleep's calculation of delay_ms. I didn't try to figure out
exactly why that's happening. It may well vary depending on the
absolute magnitude of current values of GetCurrentTimestamp(),
because I don't recall having noticed any such behavior back when
this code was written.

Anyway, I propose trying to get rid of this misbehavior by avoiding
floating point in the delay computation, as attached. With this
patch I get less surprising behavior:

regression=# \timing on
Timing is on.
regression=# do $$ begin
for i in 1..1000 loop
perform pg_sleep(0.001);
end loop; end $$;
DO
Time: 1063.997 ms (00:01.064)
regression=# do $$ begin
for i in 1..1000 loop
perform pg_sleep(0.002);
end loop; end $$;
DO
Time: 2172.849 ms (00:02.173)

The code is a little more tied to TimestampTz being measured in
microseconds than it was before, but it wouldn't really be much
harder to fix if we ever change that.

regards, tom lane