onBackpressureBufferToFile review please

The ability to buffer streams to disk has been something that I've wondered about for a while.

Can I get peoples comments/review of this new operator please?

[`Transformers.onBackpressureBufferToFile`](https://github.com/davidmoten/rxjava-extras/blob/master/README.md#transformersonbackpressurebuffertofile)

I'd love to get review of this new operator in terms of
- what use cases have _you_ got? (volume, serialized size, rate, platform, constraints)
- overall approach
- API
- correctness of code (a big one because of the numerous sections of code subject to concurrency)
- testing on different OS (currently just tested on linux)
- performance (I've favoured correctness initially)
- anything else you think of

The code is in the master branch and runtime jar is on Maven Central as described in rxjava-extras [README](https://github.com/davidmoten/rxjava-extras).

A quick way of contributing is to run a long running test (~30 mins) on your machine:

``` bash
git clone https://github.com/davidmoten/rxjava-extras.git
cd rxjava-extras
./test-long.sh
```

On non-nix platform just run this command instead of `test-long.sh`:

```
mvn clean install -Dmax.small=100000000 -Dmax.medium=300000 -Dmax.seconds=600 -Dloops=10000
```