Refactoring to use chained batch instead of array batch.

All the work being done in https://github.com/rvagg/node-levelup/issues/171 by @brycebaril shows that using chained batch is much better then using array batch.

I'm trying this for https://github.com/mcollina/levelgraph/issues/40.

However I'm getting some segfaults under heavy load.

I also got this error:
```
events.js:72
        throw er; // Unhandled 'error' event
              ^
WriteError: Corruption: WriteBatch has wrong count
    at /Users/matteo/Temp/lg-mem-leak/node_modules/level-hyper/node_modules/level-packager/node_modules/levelup/lib/batch.js:68:39
````