(#2890) - reduce md5 chunk size to 32768

This is hardly a slam dunk, but by reducing the md5 chunk size, it seems to me to block DOM rendering much less than before.

In these videos I'm loading some large-ish GIFs wholesale into WebSQL in a WebView on a Nexus 5 running Android 4.4.4. The Blobs are provided via xhr with `xhr.responseType = 'arraybuffer'`, then `new Blob()`.

[Before 1](https://dl.dropboxusercontent.com/u/2588462/pouchdb/videos/2890/before.mp4), [After 1](https://dl.dropboxusercontent.com/u/2588462/pouchdb/videos/2890/after.mp4)
[Before 2](https://dl.dropboxusercontent.com/u/2588462/pouchdb/videos/2890/before2.mp4), [After 2](https://dl.dropboxusercontent.com/u/2588462/pouchdb/videos/2890/after2.mp4)

The DOM is still obviously getting blocked, because the GIFs freeze, but they seem to freeze a bit less when the chunk size is lower. But even if it's just my imagination, can we agree that 32KB is a more reasonable default than 500KB in browsers? Or that maybe we should make this configurable?