Set FIRST_KEY_INDEX if a script accepts keys

[poiuj]

Description

Setting FIRST_KEY_INDEX ensures we send the script to the right node, instead of sending to random node and repeating on getting MOVED repsonse. Relying on a random node send increases traffic and exceeds maxCommandRedirections configuration with non-zero probability.

Checklist

• [x] Does npm test pass with this change (including linting)?
• [x] Is the new or changed code fully tested?
• [x] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)? No need for documentation update.

[poiuj]

Closed it by mistake. Opened a new one – https://github.com/redis/node-redis/pull/2858