Avro adapter: setPosition doesn't work with BaseVariableWidthVectors

### Describe the bug, including details regarding any error messages, version, and platform.

`BaseVariableWidthVector`-based vectors (`VarBinaryVector`, `VarCharVector`) expect the data to be written sequentially. This means `Consumer.setPosition` is not safe to use. Imagine the following scenario:

- write index 0
- write index 2
- write index 1
- write index 3

On the last step, `BaseVariableWidthVector.fillHoles` erases the data written for the index 2:

```java
protected final void fillHoles(int index) {
for (int i = lastSet + 1; i < index; i++) {
setBytes(i, emptyByteArray, 0, emptyByteArray.length);
}
lastSet = index - 1;
}
```

Here, index would be 3, lastSet + 1 would be 2 (1 + 1), so `setBytes(2, emptyByteArray, ...)` is called.

Not sure what the right solution is here, but at a minimum, the library shouldn't allow calling `setPosition` for Consumers using `BaseVariableWidthVector` vectors.

Start by tracing the Avro adapter's Consumer.setPosition behavior with BaseVariableWidthVector-based VarBinaryVector and VarCharVector instances. Reproduce the write sequence 0, 2, 1, 3 and add a regression test; done means positioning no longer erases previously written data or is prevented for these vectors.

Written by the indexing model from the issue text.