Make SortField immutable

[romseygeek]

Description

Sort objects are shallowly immutable, but their inner SortField implementations are not, which could cause hard-to-track down bugs. We should make SortField immutable as well.

The field, type and reverse fields can be made final with no change in API. Missing values are trickier, as they are currently mutable via a setMissingValue method. My suggestion would be to deprecate this method in 10.4, and add constructors that explicitly take missing values (which could also enhance type safety); and then remove the method in 11.0.

[romseygeek]

The one remaining non-final method is comparatorSource which is only used by custom sorts. Given the tight coupling between SortField.Type and the comparator factory methods, I wonder if it makes sense to set comparatorSource for all types and add some basic factory methods to the various different FieldComparator implementations. Then SortField.getComparator() just becomes a simple call to comparatorSource.getComparator().