AppendableRecordBatch

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

At Space and Time, we would like to make an OLTP columnar DB based on Arrow (see noisepage for precedent). Unfortunately, Arrow RecordBatches are immutable by design.

**Describe the solution you'd like**

Fortunately however, append-only and immutable are not necessarily at odds, as long as one takes immutable RecordBatch snapshots at a point-in-time of the append-only version. See [this gist](https://gist.github.com/avantgardnerio/60a53a3481f3d7844efa12346ef6f814) for a working PoC implementation:


**Describe alternatives you've considered**

1. using arrow2
2. trying some trickery with making the `MutableBuffer` work, vs preallocating ones from the immutable package

Start by reviewing the linked working PoC and the existing Arrow RecordBatch, MutableBuffer, and immutable-package APIs. Establish the required append-only snapshot semantics and tests before implementation; done means the project has an agreed scope for supporting appendable batches without violating immutable snapshots.

Written by the indexing model from the issue text.