Merge branch 'comment-batch-size' into 'master'

Batch size >1000 does not pay off

We did a small experiment to see how a full scan of the Redis keys on
gitlab.com speeds up as we increase the batch size. The values on the
right are time in seconds for a full scan (no delete operations).

```
count: 10);      284.500529021
count: 100);      86.21216934
count: 1_000);    60.931676195
count: 10_000);   60.96355610
count: 100_000);  62.378172541
```

It looks like 1,000 is a good number.

See merge request !2970

lib/tasks/cache.rake

 namespace :cache do
-  CLEAR_BATCH_SIZE = 1000 # The more the faster, but having too many can crash Ruby
+  CLEAR_BATCH_SIZE = 1000 # There seems to be no speedup when pushing beyond 1,000
   REDIS_SCAN_START_STOP = '0' # Magic value, see http://redis.io/commands/scan
 
   desc "GitLab | Clear redis cache"