Support truncate

[dbrgn]

Currently, it's not possible to delete all items in the index.

In [8]: CompanyIndex.objects.all()[:].delete()
SELECT `id`, `customer_id` FROM sphinx_company_rt

DELETE FROM sphinx_company_rt WHERE id IN (22554, 22553, 22552, 22551, 22550, 22549, 22548, 22547, 22546, 22545, 22544, 22543, 22542, 22541, 22540, 22539, 22538, 22537, 22536, 22535)

The first SELECT without a limit returns only 20 entries, so only those 20 entries are removed.

Current trunk version added support for the TRUNCATE INDEX indexname command. That could help.

Maybe it's possible to alter the function of the .delete() statement? Otherwise, could we add a .truncate() option to execute a TRUNCATE INDEX statement?