Fix calculation of page retained size for pages involving VariableWidthBlockEncoding blocks

[linzebing]

In VariableWidthBlockEncoding, when reading a block, we don't allocate a new Slice but instead return a view https://github.com/trinodb/trino/blob/master/core/trino-spi/src/main/java/io/trino/spi/block/VariableWidthBlockEncoding.java#L70-L71

This is inconsistent with other encodings and makes such blocks return augmented retained sizes. Ideally, we should create a new Slice, but get rid of the extra copy here https://github.com/trinodb/trino/blob/master/core/trino-main/src/main/java/io/trino/execution/buffer/PagesSerde.java#L224-L228. Also in ExchangeSourceReader we will be able to return a view too.

[findepi]

cc @sopel39

[sopel39]

That's my old PR (closed) where I wanted to fix the issue: prestodb/presto#10634. IIRC I've never benchmarked it

[linzebing]

#11235 resolves this