protocols/gossipsub: Add message signing and verification configuration (#1583)

This adds optional message signing and verification to the gossipsub protocol as
per the libp2p specifications.

In addition this commit:

- Removes the LruCache received cache and simply uses the memcache in it's
  place.

- Send subscriptions to all peers

- Prevent invalid messages from being gossiped

- Send grafts when subscriptions are added to the mesh

Co-authored-by: Max Inden <mail@max-inden.de>
Co-authored-by: Rüdiger Klaehn <rklaehn@protonmail.com>
Co-authored-by: Rüdiger Klaehn <rklaehn@gmail.com>

protocols/gossipsub/src/error.rs

@@ -0,0 +1,40 @@
+// Copyright 2020 Sigma Prime Pty Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+//! Error types that can result from gossipsub.
+
+use libp2p_core::identity::error::SigningError;
+
+/// Error associated with publishing a gossipsub message.
+#[derive(Debug)]
+pub enum PublishError {
+    /// This message has already been published.
+    Duplicate,
+    /// An error occurred whilst signing the message.
+    SigningError(SigningError),
+    /// There were no peers to send this message to.
+    InsufficientPeers,
+}
+
+impl From<SigningError> for PublishError {
+    fn from(error: SigningError) -> Self {
+        PublishError::SigningError(error)
+    }
+}