Switch srml-session keys to decl_storage! (#3184)

* Switch `srml-session` keys to `decl_storage!`

* Expose `DEDUP_KEY_PREFIX` in constants

* Fix test

* Bump spec version

substrate/node/runtime/src/lib.rs

@@ -75,8 +75,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 117,
-	impl_version: 117,
+	spec_version: 118,
+	impl_version: 118,
 	apis: RUNTIME_API_VERSIONS,
 };