Move offchain worker http helpers to sr-primitives. (#3733)

* Move offchain helpers to sr-primitives.

* Add some docs and fix tests.

* Refresh cargo.lock

* Bump runtime.

* Don't run offchain http example.

substrate/core/sr-primitives/src/offchain/mod.rs

@@ -0,0 +1,19 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A collection of higher lever helpers for offchain workers.
+
+pub mod http;