Manage connection to redis

[MrOrz]

Currently, 1 line bot server may have 10+ connection to redis server.

If we list connection in the production redis, we can see lots of connections coming from 1 line bot.

We should consider managing the number of redis connections via the following measures:

• https://stackoverflow.com/a/47939764
• Best practices for number of connections redis/node-redis#558 (comment)

[MrOrz]

Seems that most connection are from bull - 3 connection per instance. As we start multiple line bot instances, the connection count multiplies.