API & backend to list pads on epl instance

Introduced the ability to list all pads currently on this instance.
The pads can be retrieved via api calling ```/api/1.2.1/listAllPads```

How it works:
* Startup:
  * retrieves dataset containing all known padIDs from db from the key ```pads``` into storage
* Runtime:
  * opening pad:
     * adds padID to storage if not existing
     * saves storage to db
  * creating pad
     * adds padID to storage
     * saves storage to db
  * remove pad
     * removes padID from storage
     * saves storage to db

Limitations:
* Pads created before this feature are not listed