Relayer rewards improvements (#624)

* Document relayers fund existence and add root account.

* Introduce initialize method instead of assuming that relayer_fund_account is always required.

* cargo fmt --all

* Fix benchmarks.

* cargo fmt --all

* Fix docs for the relayer fund account.

bridges/primitives/message-lane/src/source_chain.rs

@@ -109,4 +109,12 @@ pub trait MessageDeliveryAndDispatchPayment<AccountId, Balance> {
 		reward: &Balance,
 		relayer_fund_account: &AccountId,
 	);
+
+	/// Perform some initialization in externalities-provided environment.
+	///
+	/// For instance you may ensure that particular required accounts or storage items are present.
+	/// Returns the number of storage reads performed.
+	fn initialize(_relayer_fund_account: &AccountId) -> usize {
+		0
+	}
 }