[C++][Parquet] Speed up DELTA_BINARY_PACKED decoding #51268

Describe the enhancement requested

DeltaBitPackDecoder decodes noticeably slower than the other integer encodings, for two
reasons that are both in the decoder rather than the format:

1. The prefix-sum loop reloads the frame and stores the running value on every value, because
   both members may alias the caller-owned output buffer.
2. The bit unpacker is called once per miniblock -- one call per 32 values with the default
   geometry -- even though miniblocks sharing a bit width are packed contiguously and can be
   unpacked in a single call.

Together these are 1.5x-1.7x on the DELTA_BINARY_PACKED decode benchmarks already in the tree,
on the arms where the running sum and per-call setup are a meaningful share of the work.

Component(s)

C++