database: Prevent write transaction with low disk space

[stevenroose]

I know it's kind of a hack, but it solves https://github.com/btcsuite/btcd/issues/1152.

In principle it does make sense to not try to operate with very little disk space, since it can cause all kinds of unexpected problems.

[zquestz]

@stevenroose you might want to look at https://github.com/gcash/bchd/pull/76

It would be better to use the db path so you are assured you are measuring the correct volume.

[jakesylvestre]

@jcvernaleo (as per #1530)

• Low priority
• Bug

Linked Issues: #1152