Merge policy update footgun

[guilload]

We are about to add the ability to update index configs, including their merge policies. You could start a "merge storm" by increasing the maturation period or the merge factor. Do we want to add a parameter to apply merge policy updates only to new splits.

[rdettai]

Note: #4832 starts addressing the index config update, but deliberately leaves the indexing settings (and hence merge policies) aside for a separate PR

[trinity-1686a]

afaict, #5078 implemented the merge policy update, and no specific protection was implemented yet