Write script to create a uniqush dump of PSPs and subscribers based on one or more rdb snapshots

[TysonAndre]

See https://github.com/sripathikrishnan/redis-rdb-tools

Doing this from uniqush would have potential harmful performance impacts on redis instances and the uniqush server

• If the database is sharded (e.g. twemproxy), then SCAN or KEYS wouldn't work as expected

Output formats:

1. JSON
2. sqlite on disk with schema and indices for subscribers (e.g. for fast lookup of subscriber, service, or device token/id)

Motivation:

• Ability to migrate to other implementations
• Checking if a device is subscribed already in batches (not real time, separate tickets exist for that)

Resources:

see db/*redis*.go for the way uniqush represents subscribers and PSPs in redis