Blockchain cache pruning strategy (#3395)

* blockchain cache pruning strategy

* added some internal docs to cache_pruning_strategy

* Update core/client/db/src/cache/mod.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

substrate/core/client/src/blockchain.rs

@@ -21,7 +21,6 @@ use std::sync::Arc;
 use sr_primitives::traits::{Block as BlockT, Header as HeaderT, NumberFor};
 use sr_primitives::generic::BlockId;
 use sr_primitives::Justification;
-use consensus::well_known_cache_keys;
 
 use crate::error::{Error, Result};
 
@@ -254,3 +253,18 @@ pub fn tree_route<Block: BlockT, F: Fn(BlockId<Block>) -> Result<<Block as Block
 		pivot,
 	})
 }
+
+/// A list of all well known keys in the blockchain cache.
+pub mod well_known_cache_keys {
+	/// The type representing cache keys.
+	pub type Id = consensus::import_queue::CacheKeyId;
+
+	/// A list of authorities.
+	pub const AUTHORITIES: Id = *b"auth";
+
+	/// Current Epoch data.
+	pub const EPOCH: Id = *b"epch";
+
+	/// Changes trie configuration.
+	pub const CHANGES_TRIE_CONFIG: Id = *b"chtr";
+}