Core: Use CharSequenceMap for writing unordered deletes

[aokolnychyi]

This PR migrates SortingPositionOnlyDeleteWriter to use CharSequenceMap to simplify the code. The new solution is essentially the same in terms of functionality and performance.

Before

Benchmark                                                                            Mode  Cnt  Score   Error  Units
ParquetWritersBenchmark.writeUnpartitionedClusteredPositionDeleteWriter                ss    5  1.682 ± 0.242   s/op
ParquetWritersBenchmark.writeUnpartitionedFanoutPositionDeleteWriter                   ss    5  2.949 ± 0.092   s/op
ParquetWritersBenchmark.writeUnpartitionedFanoutPositionDeleteWriterShuffled           ss    5  3.428 ± 0.089   s/op

After

Benchmark                                                                            Mode  Cnt  Score   Error  Units
ParquetWritersBenchmark.writeUnpartitionedClusteredPositionDeleteWriter                ss    5  1.608 ± 0.122   s/op
ParquetWritersBenchmark.writeUnpartitionedFanoutPositionDeleteWriter                   ss    5  2.998 ± 0.061   s/op
ParquetWritersBenchmark.writeUnpartitionedFanoutPositionDeleteWriterShuffled           ss    5  3.209 ± 0.480   s/op

[aokolnychyi]

@rdblue @szehon-ho @nastra @Fokko @amogh-jahagirdar @RussellSpitzer

[aokolnychyi]

Thank you, @amogh-jahagirdar!