Currently, collators and their alongside nodes spin up a full-scale
overseer running a bunch of subsystems that are not needed if the node
is not a validator. That was considered to be harmless; however, we've
got problems with unused subsystems getting stalled for a reason not
currently known, resulting in the overseer exiting and bringing down the
whole node.
This PR aims to only run needed subsystems on such nodes, replacing the
rest with `DummySubsystem`.
It also enables collator-optimized availability recovery subsystem
implementation.
Partially solves #1730.
s0me0ne-unkn0wn