When a bulk walk size is set to automatic, this is not always set to the optimal value

[TheWitness]

Describe the bug

When the bulk walk size is set to -1, it should auto-detect, but in the latest release, the bulk_walk_size never changes from -1 to the optimum vavelue.

To Reproduce

Steps to reproduce the behavior:

1. Update your devices to bulk walk auto-detect -1 "UPDATE host SET bulk_walk_size=-1"

2. Force a Re-Index of all devices "poller_reindex_hosts.php --id=all --qid=all --debug"

3. Note that the bulk_walk_size has not reset to the detected value "SELECT bulk_walk_size, count(*) AS hosts FROM host GROUP BY bulk_walk_size"

Expected behavior

Cacti should default the 'bulk_walk_size' to 0.

[TheWitness]

Turns out that -1 is to check the bulk walk size every polling cycle and if you set to 0, it will update to the best values on the next re-index.