Do not run unneeded subsystems on collator and its alongside node (#3061)

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.

polkadot/cli/src/lib.rs

@@ -29,7 +29,7 @@ mod error;
 pub use service::{self, Block, CoreApi, IdentifyVariant, ProvideRuntimeApi, TFullClient};
 
 #[cfg(feature = "malus")]
-pub use service::overseer::prepared_overseer_builder;
+pub use service::overseer::validator_overseer_builder;
 
 #[cfg(feature = "cli")]
 pub use cli::*;