Recently rejected cache for transaction queue (#9005)

* Store recently rejected transactions.

* Don't cache AlreadyImported rejections.

* Make the size of transaction verification queue dependent on pool size.

* Add a test for recently rejected.

* Fix logging for recently rejected.

* Make rejection cache smaller.

* obsolete test removed

* obsolete test removed

* Construct cache with_capacity.

ethcore/src/verification/mod.rs

@@ -42,12 +42,6 @@ pub enum VerifierType {
 	Noop,
 }
 
-impl Default for VerifierType {
-	fn default() -> Self {
-		VerifierType::Canon
-	}
-}
-
 /// Create a new verifier based on type.
 pub fn new<C: BlockInfo + CallContract>(v: VerifierType) -> Box<Verifier<C>> {
 	match v {