add options to disable range merging of vectored io

I'm seeing test failures in my PARQUET-2171 pr because assertions about the #of bytes read isn't holding -small files are being read and the vector range merging is pulling in the whole file.

```
[ERROR] TestInputOutputFormat.testReadWriteWithCounter:338 bytestotal != bytesread expected:<5510> but was:<11020>
```

I think for parquet i will add an option to disable vector io, but really the filesystems which support it should allow for merging to be disabled