Change IndexPack to use ObjectIdSubclassMap instead of ObjectIdMap

Change IndexPack to use ObjectIdSubclassMap instead of ObjectIdMap

The ObjectIdSubclassMap performs slightly better on average than the
ObjectIdMap, and uses a lot less memory as we don't need to allocate
a tree node per tracked object.

This reduces the memory footprint of IndexPack, by storing a linked
list of the UnresolvedDelta objects directly in each map, which is
slightly smaller than using an ArrayList inside of a TreeMap.

Performance is marginally faster, but we now can declare ObjectIdMap
to be fully deprecated and start removing it from the library.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>