Codegen dispatcher: avoid copying and hashing the serialized expression on every batch #6705

What is the problem the feature request solves?

On every batch, CometScalaUDFCodegen.evaluate copies the serialized bound expression out of its first argument (exprVec.get(0)), about 7 KB for a simple Scala UDF, and hashes the copy into the kernel cache key. The perf-cache-key TODO on CometScalaUDFCodegen.CacheKey already describes this.

@mbutrovich measured it on #6697 (comment). He found a fixed cost of about 8 microseconds per batch in the dispatcher, about 4 ms over 4M rows at batch size 8192, which grows as batches get smaller. About 6 microseconds of it is the difference between calling CometScalaUDFCodegen.evaluate and calling its generated kernel directly, which is where the copy and the hash happen.

Describe the potential solution

Compute a hash, or another identifier for the expression, on the driver and ship it through the JvmScalarUdf proto, so that a batch looks the kernel up without copying or hashing the closure. The TODO lists other options: per-instance memoization of the last key, or a two-tier cache keyed on the generated source.

Additional context

The measurements and the benchmark source are in the comment linked above.