Improve subdocument counter methods

• Add new versions of `MutateInSpec.incrementAndGet` and `decrementAndGet` without a `delta` parameter ("increment/decrement" implies the delta is 1).

• Add an `addAndGet` method that takes a delta.

• Deprecate the old increment/decrement methods that take a delta, in favor of addAndGet.

This aligns the subdocument counter methods with Java's AtomicLong methods, and side-steps the problem of what to do if the user attempts to subtract Long.MIN_VALUE. (The current implementation of decrementAndGet that takes a delta is flawed because it naively negates the delta and then delegates to incrementAndGet. This fails if the delta is `Long.MIN_VALUE`, because negating `Long.MIN_VALUE` has no effect.)