Ataki/gridfs

@stvstnfrd to review commit
Looking over this, I'm inclined to say we should make this
`maxChunkSize` separately configurable (instead of setting it to the
same value as `maxSize`, as is done currently).

For our use case, having these values each set to 10MB is fairly
reasonable, especially considering that this is a decrease from 100MB
each. But what if we wanted to upload a 10MB file into 1MB chunks?

On the other end of the spectrum, imagine someone explicity allowing
large uploads, say 1GB. We probably shouldn't assume the chunkSize is
still the same value, uploading into a 1GB chunk in GridFS.