Small bit of simplification

[philjdf]

1. Noticed an unused parameter elementType in the constructor of LogicalColumnStream, and also that the Buffer etc members of that class are unused except at the leaf subclass levels, so this looks like it should be composition rather than inheritance.
2. Noticed LogicalColumnReader<TPhysical, TLogical, TElement> has TPhysical and TLogical generic type args only being used at construction time, so move those types to a static factory method. Also rename it to LogicalColumnReaderBatch to avoid the clash with the base class.
3. Now there doesn't seem much point in having LogicalColumnReaderBatch, merge it with its parent class LogicalColumnReader. So instead of having sealed LogicalColumnReader<TPhysical, TLogical, TElement> : abstract LogicalColumnReader<TElement> : abstract LogicalColumnReader we've got sealed LogicalColumnReader<TElement> : abstract LogicalColumnReader.
4. Same on the LogicalColumnWriter side.
5. Move the LogicalColumnStreamBuffer creation inside the LogicalBatchReader/WriterFactory classes, and it turns out there's already a LogicalStreamBuffers<TPhysical> class there which we can use instead.

[adamreeve]

This looks great thanks Phil!