Rework backend to use multiple redis connections for tree view and CLI

[sseide]

Current implementation uses only one connection per configured redis server. This connection is shared between the tree view operations and CLI commands for all users. Therefor its not working well with multiple commands used via CLI:

• select (no way to rename keys #378)
• multi (Can't use MULTI? #337)
• pub/sub (When Subscribing: "ERROR: Connection in subscriber mode, only subscriber commands may be used" #368)

To fix these issues the connection handling must be rewritten to use multiple connections per database.

• Main connection list may be used as is - but only for operations related to the UI tree view (fetch folder and keys for display on upper part of UI) (=> base connection). These one connect automatically on server start.
• On first use of CLI new connection is cloned from base connection (ioredis duplicate()) and used for all CLI commands only (lazy initialize)
• must introduce kind of user session handling to have one redis connection per user per CLI.
  Connections must be closed if user session expires/not used anymore. Very important for pub/sub and multi!

If implemented increment minor version number of RC.