parallelize streams

[mccanne]

For queries that are flow insensitive, the same parallelization logic that we use for lake can also apply to streams (e.g., zq or internal readers like ingest).

We should logic to the optimizer to handle this case. The same logic that applies to lister/scanner parallelization can easily adapt to kernel.Reader.

[nwt]

#4920 replaced kernel.Reader with dag.DefaultScan, to which this issue still applies.