Possible bug in kNN with mahalanobis metric

@VesnaT, I have put the code that manipulated the learner's params into `_initialize_wrapped` to avoid change the learner's parameters when fitting -- the same learner might be shared by multiple threads. Is your code (from 2016, I'm sure you still remember it :) that manipulates `metric_params` safe for multithreading?

_Originally posted by @janezd in https://github.com/biolab/orange3/issues/7309#issuecomment-5070155819_

Start by tracing the kNN implementation's handling of the mahalanobis metric and metric_params, then compare that behavior with the parameter manipulation moved into _initialize_wrapped. Determine whether shared learners can be mutated during fitting and verify the expected behavior for concurrent use; the issue names no files or tests.

Written by the indexing model from the issue text.