Wrong order in BalancerStrategy.pickServersToDrop()

`ServerHolder` are sorted by *available* space, so `NavigableSet` (unless uses `Comparator.reverseOrder()`) have ServerHolders with the *most* available space in the end. Therefore `set.descendingIterator()` suggests to drop segment from least utilized servers first, that's the opposite of what should be done.

However, I think that this default implementation should just be removed because `BalancerStrategy` is not annotated `@ExtensionPoint` and all production implementations of `BalancerStrategy` override this method. Then the signature should be simplified too, accepting any `Collection` rather than only `NavigableSet`.

FYI @clintropolis

Start at BalancerStrategy.pickServersToDrop() and inspect how its NavigableSet ordering and descendingIterator() select servers. Review all production implementations that override the method and determine the impact of changing the parameter to Collection. Done means the incorrect default behavior is removed and the affected implementations and callers remain consistent.

Written by the indexing model from the issue text.