[batchprocessor] Modernize or remove the Batch processor

[jmacd]

Describe the bug
A number of known defects have been allowed to stay in the batchprocessor, while there has been a debate over whether to remove it or upgrade it. Since exporterhelper now supports all of the same features, we could remove batchprocessor. It was suggested that it would be less disruptive to simply upgrade it to use the newer logic from exporterhelper.

Part of #13766

The defects:

• suppression of errors (never returns an error)
• early return before request has completed (unconditional)
• lack of concurrency

I have put together a change that begins to address these -- the first two bullets for now behind two feature flags. The third (concurrency) could be easily added, too.

The only major obstacles to completing this are:

• [exporterhelper] Add an ability to batch by a key from pdata or context #10825 (metadata_key batching, not completely finished in exporterhelper)
• The metrics emitted by exporterhelper-based-batcher are incorrect, because they are named for exporters.

[jmacd]

#13478 is a prerequisite.