Failed locking attempts were immediately retried up to three times
without any delay between the retries. If a lock file is not found while
checking for other locks, with the reworked backend retries there is no
delay between those retries. This is a problem if a backend requires a
few seconds to reflect file deletions in the file listings. To work
around this problem, introduce a short exponentially increasing delay
between the retries. The number of retries is now increased to 4. This
results in delays of 5, 10 and 20 seconds between the retries.
