Avoid copying strings from dictionary or plain-encoded blocks

When scanning a plain or dictionary-encoded binary column, we currently loop over each entry and copy the string into the destination RowBlock's arena. In TPCH Q1, the scanner threads use a significant percentage of CPU doing this copying, and it also increases CPU cache footprint which likely decreases performance in downstream operations like predicate evaluation, merging, result serialization, etc.

Instead of doing this, we could "attach" the dictionary block (with ref-counting) to the RowBlock and refer directly to the dictionary entry from the RowBlock. When the RowBlock eventually is reset, we can drop the reference. This should be safe because we never mutate indirect data in-place.