gc.sh: attempt to recover from a stale gc.pid.lock file
Under normal conditions a gc.pid.lock file should exist
only briefly for much less than a second. On the other
hand, if failures occur (i.e. disk full) the gc.pid.lock
file can end up being left around. Attempt to recover
from this situation when the gc.pid.lock file is stale
(i.e. over 1 hour old) by removing it and then sleeping
for a brief period to compensate for the possible race
condition between testing for staleness and then removing
that stale file.