Missing import in 035_standby_logical_decoding.pl

Hi,

035_standby_logical_decoding.pl has this code

# wait for postgres to terminate
foreach my $i (0 .. 10 * $PostgreSQL::Test::Utils::timeout_default)
{
    last if !-f $node_standby->data_dir . '/postmaster.pid';
    usleep(100_000);
}

but it does not import usleep as other tests do (like this)
use Time::HiRes qw(usleep);

I think it simply hasn't been called because postgres exits and
terminates the loop before it has had a chance to be called. If you
reorder the sleep and the loop termination condition, it fails
immediately because it can't find usleep.

On an unrelated note, I also noticed that hot_standby_feedback is not
turned on during the test until we are explicitly checking for
recovery conflicts. When it is enabled, the comment says "Turn
hot_standby_feedback back on" -- but it was never on to begin with. I
suspect this doesn't produce test instability because autovacuum is
turned off for the whole test. But, I was wondering if the test
authors meant for hot_standby_feedback to be enabled during the
initial phases of the test (until it is explicitly disabled for
testing purposes).

- Melanie