[CASSANDRA-17298][4.0] Improve accuracy of memtable heap usage tracking

[netudima]

Fix estimateRowOverhead in Memtable: ByteBuffer in Partition key was not counted Fix non-counted size of "columns" field in Columns.unsharedHeapSize BTree.sizeOfStructureOnHeap wrongly reported a non-zero size for EMPTY_LEAF shared constant value Fix MemtableSizeTest flakeness by not measuring of allocator and ColumnFamilyStore/Keyspace

Patch by Dmitry Konstantinov; reviewed by TBD for CASSANDRA-17298