Verify signature and session index during apply phase of im-online. (#3418)

* Verify signature and session index during apply phase of im-online.

* Bump impl_version.

* Add docs to SignedExtension

substrate/node/runtime/src/lib.rs

@@ -81,7 +81,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 144,
-	impl_version: 144,
+	impl_version: 145,
 	apis: RUNTIME_API_VERSIONS,
 };