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/network/bitfield-distribution/src/lib.rs

@@ -179,6 +179,9 @@ impl BitfieldDistribution {
 						// defer the cleanup to the view change
 					}
 				}
+				FromOverseer::Signal(OverseerSignal::BlockFinalized(hash)) => {
+					trace!(target: "bitd", "Block finalized {:?}", hash);
+				}
 				FromOverseer::Signal(OverseerSignal::Conclude) => {
 					trace!(target: "bitd", "Conclude");
 					return Ok(());