Add VariableWidthRandomAccessBlockCursor
Remove VariableWidthBlock and VariableWidthBlockCursor
Remove unused PoissonizedBlock
Change SnappyBlock to be random access
Merge RandomAccessBlock and Block
Merge RandomAccessPage and Page
Rename VariableWidthRandomAccessBlock to VariableWidthBlock
Make ChannelSet immutable

Rewrite ChannelSet using GroupByHash as a template