Fixed incorrect counter increments to potentially atomic operations.

Earlier version set new expiration on every request. For example,
if the limit is 1 request/minute, and client performs one request
every 59 seconds, after first one, no request is allowed ever, as
the counter expiration is reset after each (denied) request.

Unfortunately, Django cache.incr checks whether key exists, even
though (at least) redis creates the key automatically. This
causes an additional, unnecessary EXISTS query for each INCRBY
command.