chore: upgrade level* components

[rvagg]

leveldown@5 is out and will deal with failures in next month's Node 12 (and warnings with Node 11)

[vweevers]

Be advised, this changes the minimum node version to 8.6.0. I had a quick look at src/index.js, it's not affected by other breaking changes AFAICT.

Side note: I think the query() method which currently does a "full scan" can be optimized by using db.iterator({ gte: .. }) instead of:

js-datastore-level/src/index.js

Lines 175 to 178 in 7cc4144

	if (q.prefix != null) {
	const prefix = q.prefix
	filters.push((e, cb) => cb(null, e.key.toString().startsWith(prefix)))
	}

[rvagg]

minimum node version to 8.6.0

FYI 6 is EOL next month and 8.6.0 was 18 months ago, so in compatibility terms we're on pretty good ground with this upgrade I think.

[alanshaw]

@pgte can we please get this merged and released now that Node.js 12 is out? 🙏

[pgte]

Landed on v0.11.0