ARROW-2225: [JS] support tables split across buffers

[trxcllnt]

@TheNeuralBit somewhere along the way we lost this. Resolves https://issues.apache.org/jira/browse/ARROW-2225

[wesm]

Any way to test this?

[TheNeuralBit]

@wesm I'm working on testing it right now, I can merge when I'm done

Edit: If you mean automated tests - we used to have snapshot files that exercised this feature but not anymore after switching over to integration tests

[TheNeuralBit]

LGTM - Tested locally with an Arrow stream I manually split into a "Schema" and a "Batches" buffer.

We should definitely add a test for this, but I'm not sure how to do it right now.. any objections to waiting until we have a JS IPC writer?

[trxcllnt]

@TheNeuralBit yeah, that's what I was thinking too. I think we can sit tight since that's now back in progress.

[TheNeuralBit]

Created ARROW-2235 to track test creation