Support separators in doc_id

[kaapstorm]

Context: SC-3872

This change accommodates merged payloads from CommCare HQ.

The value of "doc_id" of a merged payload is a comma-separated list of the doc_ids it contains. e.g.

{
  "data_source_id": "abc123",
  "doc_id": "a1,b2",
  "data": [
    {"doc_id": "a1", "foo": "bar"},
    {"doc_id": "b2", "foo": "baz"}
  ]
}

[kaapstorm]

Commit 6aa8a65 supports the new payload format introduced in PR dimagi/commcare-hq#35732 while still supporting the current payload format.