an exclusive lock on the table at this point, which we want to release as soon
as possible.  This is called in the phase of lazy vacuum where we truncate the
empty pages at the end of the table.

An alternative solution would be to lower the vacuum delay settings before
starting the truncating phase, but this doesn't work very well in autovacuum
due to the autobalancing code (which can cause other processes to change our
cost delay settings).  This case could be considered in the balancing code, but
it is simpler this way.