Remove lifetime from DynComparator

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The cost of building a comparator (initialising a DynComparator) is often significantly higher than the actual cost of executing the comparator's closure on two row IDs. Therefore it makes sense to build the comparator once, and re-use the returned DynComparator for each row you are comparing the two arrays on.

Due to the explicit lifetime of DynComparator I recently found it problematic to store the DynComparator on an object that was used across threads in an async environment.

Describe the solution you'd like
Remove lifetime from DynComparator