Change in probing behaviour.

If SyzygyProbeLimit is 6 (the default), then:
- if available, 6-piece tables are probed in nodes with remaining depth >= SyzygyProbeDepth;
- 3/4/5-piece tables are probed without depth limit.

If SyzygyProbeLimit is 5, then:
- 6-piece tables are not probed;
- 5-piece tables are probed in nodes with remaining depth >= SyzygyProbeDepth;
- 3/4-piece tables are probed without depth limit.

Unless your machine has very limited RAM (i.e. no room for caching 400 MB of 3/4/5-piece tables), leave SyzygyProbeLimit at the default. Also if you don't have 6-piece tables.