Fix more edge cases in autopipelining when using redis cluster

[TysonAndre]

Fixes #1392

This is a superset of the fixes in #1391

The other issue this PR fixes was that this was passing an array instead of a string to
cluster-key-slot when args[0] was an array

Handle pathological cases such as redis.get([], ['key1', 'key2'])
by flattening the entire args array.

Prior to this PR, in the below example, the array was hashed instead of the first string in the array,
meaning that 'v{profile}1' and 'b' were incorrectly put in the same pipeline

const IORedis = require('./built');
const process = require('process');
async function main() {
    // Using instructions from https://redis.io/topics/cluster-tutorial
    // redis-cli --cluster create 127.0.0.1:7000 127.0.0.1:7001 127.0.0.1:7002 --cluster-replicas 0
    const redis = new IORedis.Cluster([{ host: '127.0.0.1', port:7000 }], { enableAutoPipelining: true });
    const results = await Promise.all([
      redis.mget(['v{profile}1', 'v{profile}2', 'v{profile}3', 'v{profile}4']),
      redis.get('b'),
    ]);
    console.log(results);
    process.exit(0);
}
main();

[TysonAndre]

Checking node_modules/redis-commands/commands.json for keyStart == keyStop may be a cheap way to detect commands such as get which have only one key without needing to flatten a potentially large array, but it seems like the redis-commands API doesn't treat commands.json as part of the public API. Not sure if that would be micro-optimizing too much.

Alternately, the multiple key check could just be omitted, since that would be a redis cluster sharding error anyway already