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/snapshot/error.rs

@@ -59,6 +59,10 @@ pub enum Error {
 	ChunkTooSmall,
 	/// Snapshots not supported by the consensus engine.
 	SnapshotsUnsupported,
+	/// Bad epoch transition.
+	BadEpochProof(u64),
+	/// Wrong chunk format.
+	WrongChunkFormat(String),
 }
 
 impl fmt::Display for Error {
@@ -82,6 +86,8 @@ impl fmt::Display for Error {
 			Error::VersionNotSupported(ref ver) => write!(f, "Snapshot version {} is not supprted.", ver),
 			Error::ChunkTooSmall => write!(f, "Chunk size is too small."),
 			Error::SnapshotsUnsupported => write!(f, "Snapshots unsupported by consensus engine."),
+			Error::BadEpochProof(i) => write!(f, "Bad epoch proof for transition to epoch {}", i),
+			Error::WrongChunkFormat(ref msg) => write!(f, "Wrong chunk format: {}", msg),
 		}
 	}
 }