[C++] Disable simdjson threading in chunker #51471

Describe the enhancement requested

When invoking simdjson's parse_many, it uses threading to perform its stage 1 parsing on the next batch when while parsing the current batch:
https://github.com/simdjson/simdjson/blob/master/doc/parse_many.md?plain=1#L96-L104

In most cases, we do not need to parse any batches after the first one to find delimiters (in fact, we currently would even error, when the first batch does not contain the whole first document). By disabling the parser's threading we can expect performance improvements

Component(s)

C++