Move authorities interface from Core to consensus (#1412)

* Move authorities interface from Core to consensus

f

* notify all caches of block insert + create with up-to-date best_fin

* merged authorities_are_cached from light_grandpa_import2

* Add ProvideCache trait

* Create helper function for 'get_cache'

* Fix some formatting

* Bump impl version

* Resolve wasm conflicts

* Apply review comments

* Use try_for_each

* Move authorities interface from Core to consensus

f

* notify all caches of block insert + create with up-to-date best_fin

* merged authorities_are_cached from light_grandpa_import2

* Add ProvideCache trait

* Create helper function for 'get_cache'

* Fix some formatting

* Bump impl version

* Resolve wasm conflicts

* Apply review comments

* Use try_for_each

* Move authorities interface from Core to consensus

f

* notify all caches of block insert + create with up-to-date best_fin

* merged authorities_are_cached from light_grandpa_import2

* Add ProvideCache trait

* Create helper function for 'get_cache'

* Fix some formatting

* Bump impl version

* Resolve wasm conflicts

* Apply review comments

* Use try_for_each

* Increment impl_version

* Update lib.rs

substrate/core/client/src/blockchain.rs

@@ -16,7 +16,9 @@
 
 //! Substrate blockchain trait
 
-use runtime_primitives::traits::{AuthorityIdFor, Block as BlockT, Header as HeaderT, NumberFor};
+use std::sync::Arc;
+
+use runtime_primitives::traits::{Block as BlockT, Header as HeaderT, NumberFor};
 use runtime_primitives::generic::BlockId;
 use runtime_primitives::Justification;
 
@@ -78,7 +80,7 @@ pub trait Backend<Block: BlockT>: HeaderBackend<Block> {
 	/// Get last finalized block hash.
 	fn last_finalized(&self) -> Result<Block::Hash>;
 	/// Returns data cache reference, if it is enabled on this backend.
-	fn cache(&self) -> Option<&Cache<Block>>;
+	fn cache(&self) -> Option<Arc<Cache<Block>>>;
 
 	/// Returns hashes of all blocks that are leaves of the block tree.
 	/// in other words, that have no children, are chain heads.
@@ -89,10 +91,16 @@ pub trait Backend<Block: BlockT>: HeaderBackend<Block> {
 	fn children(&self, parent_hash: Block::Hash) -> Result<Vec<Block::Hash>>;
 }
 
+/// Provides access to the optional cache.
+pub trait ProvideCache<Block: BlockT> {
+	/// Returns data cache reference, if it is enabled on this backend.
+	fn cache(&self) -> Option<Arc<Cache<Block>>>;
+}
+
 /// Blockchain optional data cache.
 pub trait Cache<Block: BlockT>: Send + Sync {
-	/// Returns the set of authorities, that was active at given block or None if there's no entry in the cache.
-	fn authorities_at(&self, block: BlockId<Block>) -> Option<Vec<AuthorityIdFor<Block>>>;
+	/// Returns cached value by the given key.
+	fn get_at(&self, key: &[u8], block: &BlockId<Block>) -> Option<Vec<u8>>;
 }
 
 /// Blockchain info