Feature/spend coconut (#1261)

* Add coconut verifier structure for coconut protocol in gateway

* Add endpoint for validator-api cred verification

* Remove unused signature field

* Register new endpoint

* Improve validator-api config handling

* Aggregate verif result from all apis

* Simplify aggregate functions

* Verify cred on apis correctly

* Introduced coconut bandwidth contract to validator client

* Fix rebase double import

* Fix clippy on non-coconut

* Add multisig contract address to validator client

* Refactor Credential struct

* Do bincode magic in the coconut interface

* Implement serialization for credential and remove bindcode

* Fix clippy and don't remove dkg

* Client release funds proposal

* Add wrapper for blinded serial number

Also compare theta with a blinded serial number (in base 58 form)
for future double spend protection.

* Only post blinded serial number to blockchain

* Validator api propose credential spending

* Fix wallet

* Gateway calls proposal creation

* Query for proposal in verify coconut

* Remove db from git

* Verify against proposal description

* Validator apis vote based on verification of cred

* Fix wallet fmt

* Execute the release of funds

* Fix translation between token and bytes

* Update CHANGELOG

mixnode/src/node/mod.rs

@@ -244,14 +244,18 @@ impl MixNode {
         atomic_verloc_results
     }
 
-    // TODO: ask DH whether this function still makes sense in ^0.10
-    async fn check_if_same_ip_node_exists(&mut self) -> Option<String> {
+    fn random_api_client(&self) -> validator_client::ApiClient {
         let endpoints = self.config.get_validator_api_endpoints();
         let validator_api = endpoints
             .choose(&mut thread_rng())
             .expect("The list of validator apis is empty");
-        let validator_client = validator_client::ApiClient::new(validator_api.clone());
 
+        validator_client::ApiClient::new(validator_api.clone())
+    }
+
+    // TODO: ask DH whether this function still makes sense in ^0.10
+    async fn check_if_same_ip_node_exists(&mut self) -> Option<String> {
+        let validator_client = self.random_api_client();
         let existing_nodes = match validator_client.get_cached_mixnodes().await {
             Ok(nodes) => nodes,
             Err(err) => {