Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/bin/node/cli/Cargo.toml

@@ -60,6 +60,7 @@ sp-keystore = { path = "../../../primitives/keystore" }
 sp-consensus = { path = "../../../primitives/consensus/common" }
 sp-transaction-storage-proof = { path = "../../../primitives/transaction-storage-proof" }
 sp-io = { path = "../../../primitives/io" }
+sp-mixnet = { path = "../../../primitives/mixnet" }
 sp-statement-store = { path = "../../../primitives/statement-store" }
 
 # client dependencies
@@ -82,6 +83,7 @@ sc-service = { path = "../../../client/service", default-features = false}
 sc-telemetry = { path = "../../../client/telemetry" }
 sc-executor = { path = "../../../client/executor" }
 sc-authority-discovery = { path = "../../../client/authority-discovery" }
+sc-mixnet = { path = "../../../client/mixnet" }
 sc-sync-state-rpc = { path = "../../../client/sync-state-rpc" }
 sc-sysinfo = { path = "../../../client/sysinfo" }
 sc-storage-monitor = { path = "../../../client/storage-monitor" }