Utilising BTree Improvements

This patch utilises CASSANDRA-15510 to improve throughput and reduce garbage produced by a number of common operations, by employing transformAndFilter, transform and FastBuilder

• Row, Cell and ComplexColumnData cloning are implemented with BTree.transform, so no special builders are necessary;
  • Rows.copy removed
• Rows.merge implemented using BTree.update and a ColumnData reconciler
  • Zero-allocations if result of merge is same as existing
  • Fewer comparisons
• ColumnData reconciler implemented in same manner
  • Cells.reconcileComplex is retired
  • ComplexColumnData reconciliation now
    • Garbage-free if the merge has no effect
    • Always fewer allocations
    • Fewer comparisons
• FastBuilder employed widely:
  • ClusteringIndexNamesFilter deserialization
  • Columns deserialization
  • PartitionUpdate deserialization
  • AbstractBTreePartition construction
  • Misc others

The upshot of this work when combined with the proposed patch for CASSANDRA-15367 has a dramatic impact on operations over collection types - under contention, as much as 100x improved throughput, and hundreds of megabytes of reduced allocations. For all operations, allocations under contention and no contention are significantly reduced and throughput improved.