Runtime storage sync should use any storage node

[kostko]

Currently storage node sync only uses nodes from the current storage committee. Instead, it should be also able to use (maybe with a lower priority) other storage nodes registered for the runtime to increase availability as certain runtime configurations (e.g., using a storage committee of size 1) make it more likely that the active committee does not have the required roots.

This should be backwards compatible and backported to the 20.11.x stable branch.