Never fill the disk

We should stop writing to the datastore once we get down to some minimum amount of remaining space left. Unfortunately, this'll be tricky due to platform differences but go-ipfs really stops behaving well when the disk fills up.

Start by tracing the datastore write paths in go-ipfs and identifying how remaining disk space can be measured on the supported platforms. Define the minimum-space threshold and verify that datastore writes stop before the disk fills, including platform-specific behavior.

Written by the indexing model from the issue text.