support both batch and single mode for LengthDelimitedStream

[eriktaubeneck]

          one of the big challenges I've had thus far is how to reason about `enc_reports` and how it could interact with reshard. but collection all of these into a stream solves that and makes it much easier to deal with.

I wonder if we should consider hiding some of that complexity "under the hood" in LengthDelimitedStream and have it operate like a "normal" stream that where .next() gives an individual item, and not a Vec. not in scope for this PR, but curious of others opinions here.

Originally posted by @eriktaubeneck in #1362 (comment)