Proposal: Add `db.has(key)` and `db.hasMany(keys)`

[sebastianst]

I love working with the Level stack. However, I miss one feature: a way to check any db whether it has a value for a key, without actually retrieving the value. Right now, as a workaround, I extend LevelUp instances with has and hasMany keys like so (TypeScript):

	db.has = async function (
		key: K,
		options?: AbstractOptions,
	): Promise<boolean> {
		try {
			await this.get(key, options);
			return true;
		} catch (e) {
			if (e.notFound) {
				return false;
			} else {
				throw e; // rethrow other exceptions
			}
		}
	};

// similar for hasMany using getMany mapped with `x => !(x === undefined)`

However, this is not as efficient as it could be, since the underlying storage can probably implement has much more efficient than first actually retrieving the value, just for it to be abandoned in this workaround.

This could even be implemented in levelup first usind above code, if levelup itself cannot find functions has or hasMany on the abstract-leveldown passed to it. This way, leveldown implementations can one by one add has/hasMany while levelup can already provide the functionality in a backwards-compatible fashion.