* Improve performance of partitionByKey with a big number of elements
* Improve performance of partitionByKey by not serialising the keys
* partition by key to emit deltas instead of the resulting keys
* update tests for partitionByKey
* update tests for combineKeys
* getGroupedObservable: close outer subscription once the inner stream is found
* add performance tests to partitionByKey
* make tests more predictable
* adjust performance threshold
Co-authored-by: Josep M Sobrepere <jm.sobrepere@gmail.com>