PoA warp implementation (#5488)

* separate modules for consensus snapshot chunks

* bulk of authority warp logic

* finish authority warp implementation

* squash warnings and enable authority snapshot mode

* test harness for PoA

* fiddle with harness

* epoch generation proof fixes

* update constructor code

* store epoch transition proof after block commit

* basic snap and restore test

* use keyvaluedb in state restoration

* decompress chunks

* fix encoding issues

* fixed-to-contract-to-contract test

* implement ancient block import

* restore genesis transition in PoW snapshot

* add format version method to snapshot components

* supported version numbers in snapshot_components

* allow returning of ancient epoch transitions

* genesis hash mismatch check

* remove commented code

ethcore/src/engines/basic_authority.rs

@@ -216,6 +216,10 @@ impl Engine for BasicAuthority {
 	fn sign(&self, hash: H256) -> Result<Signature, Error> {
 		self.signer.sign(hash).map_err(Into::into)
 	}
+
+	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
+		Some(Box::new(::snapshot::PoaSnapshot))
+	}
 }
 
 #[cfg(test)]