Availability store subsystem (#1404)

* Initial commit

* WIP

* Make atomic transactions

* Remove pruning code

* Fix build and add a Nop to bridge

* Fixes from review

* Move config struct around for clarity

* Rename constructor and warn on missing docs

* Fix a test and rename a message

* Fix some more reviews

* Obviously failed to rebase cleanly

polkadot/node/subsystem/src/lib.rs

@@ -84,6 +84,8 @@ impl PartialEq for ActiveLeavesUpdate {
 pub enum OverseerSignal {
 	/// Subsystems should adjust their jobs to start and stop work on appropriate block hashes.
 	ActiveLeaves(ActiveLeavesUpdate),
+	/// `Subsystem` is informed of a finalized block by its block hash.
+	BlockFinalized(Hash),
 	/// Conclude the work of the `Overseer` and all `Subsystem`s.
 	Conclude,
 }