Sensible way of selecting Prime member (#5346)

* Calculate prime votes only during the election

* Migration

* Fix build, enable migration

* Fix tests

* Bump runtime version

* Update frame/elections-phragmen/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

Co-authored-by: Marcio Diaz <marcio.diaz@gmail.com>

substrate/bin/node/runtime/src/lib.rs

@@ -83,7 +83,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to 0. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 239,
+	spec_version: 240,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 };