Optimize memory usage by AtomicUpdateGroup

There is a comment inside `AtomicUpdateGroup` talking about the importance of memory usage by this class.

The Javadoc comment for the class also says that this object usually contains just one entry.

If so, the easy optimization is to initialize `chunks = new ArrayList<>(1)` - with the capacity of 1.

A more advanced version would be for `OvershadowableManager` to store `Short2ObjectSortedMap` maps, where the values may be either `PartitionChunk` directly (if there is one chunk in a group), or `AtomicUpdateGroup` (if there are 2+ chunks in a group). It would complicate ` OvershadowableManager`'s code but would allow avoiding a level of indirection and creation of a lot of `AtomicUpdateGroup`, `ArrayList`, and `Object[]` objects.

FYI @jihoonson

Search for AtomicUpdateGroup and OvershadowableManager, then read the class comment and the existing memory-usage comment to understand the intended representation. Decide which of the proposed optimizations is in scope; done means the selected memory reduction is implemented without changing update-group behavior and is validated by the relevant tests or measurements.

Written by the indexing model from the issue text.