Key files include timestamp in name. (#1700)

* Key files include timestamp in name.

Introduce timestamp into new key files; keep filename around, so
that we don't accidentally duplicate keys.

* Remove unnecessary clone

* Fix test code.

* Remove log module from ethstore

ethstore/tests/util/transient_dir.rs

@@ -64,7 +64,7 @@ impl KeyDirectory for TransientDir {
 		self.dir.load()
 	}
 
-	fn insert(&self, account: SafeAccount) -> Result<(), Error> {
+	fn insert(&self, account: SafeAccount) -> Result<SafeAccount, Error> {
 		self.dir.insert(account)
 	}