Batch solr json writer

Replace `solr_json_writer` with a batched version.

Changes:
  * move old `solr_json_writer` to `nobatch_solr_json_writer`
  * use of new class `Traject::AllRubyThreadPool`l under MRI (by @jrochkind)
  * use of new class `Traject::Queue` under MRI (wanted a safe `#drain_to`)
  * use `Concurrent::AtomicFixed` under both (on JRuby, it falls back to the same java.util.concurrent class we were using before, so no performance loss)

Not at all sure how to test any of this.