Committed the patch.

See: 93832cec4c31

The best I could for testing was to run:

./run_tests.py -k Xapian test/test_indexer.py &
./run_tests.py -k Xapian test/test_indexer.py

and confirmed that one of the processes seemed to hang on a test and
then threw a lock failure error.

If anybody knows how to test this and do controlled parallel
process invocation under pytest, I would love to figure out
how to code it.

I decided not to add a user configurable delay/number of cycles.
Hopefully the 10 cycles in the code will do the trick. If not
I suspect we need a better way to handle the error than a retry.
Simply increasing the number of cycles is not the way to do it.