Make all partitions btree backed

Following on from the other btree related refactors, this patch makes all partition (and partition-like) objects backed by the same basic structure: AbstractBTreePartition. With two main offshoots: ImmutableBTreePartition and AtomicBTreePartition

The main upshot is a 30% net code reduction, meaning better exercise of btree code paths and fewer new code paths to go wrong. A secondary upshort is that, by funnelling all our comparisons through a btree, there is a higher likelihood of icache occupancy and we have only one area to focus delivery of improvements for their enjoyment by all.