Index ingest batch sizes should be configurable

In the code today, there's...

```
const BLEVE_DEST_INITIAL_BUF_SIZE_BYTES = 20000
const BLEVE_DEST_APPLY_BUF_SIZE_BYTES = 200000
```

Either we should have some configurability, or (fancier, not sure if better) maybe some self-tuning: start off favoring initial data loading, then it automagically works it way down to favoring ingest latency... and, then back up again once there's more mass data loading.

Link to related issue: https://github.com/couchbaselabs/cbft/issues/11 but have configurability as a separate issue here just in case batch sizes turn out somehow to be not the issue.