Random instance selection (#136)

* wip

* revert some'

* revert more

* poor-man's integration test

* remove test

* fmt

* --workspace

* fix build

* fix integration test

* another stab

* log

* run after integration

* cargo test after integration

* revert

* revert more

* Refactor + clean up

* more clean up

src/client.rs

@@ -499,7 +499,6 @@ where
         // The query router determines where the query is going to go,
         // e.g. primary, replica, which shard.
         let mut query_router = QueryRouter::new();
-        let mut round_robin = rand::random();
 
         // Our custom protocol loop.
         // We expect the client to either start a transaction with regular queries
@@ -631,12 +630,7 @@ where
 
             // Grab a server from the pool.
             let connection = match pool
-                .get(
-                    query_router.shard(),
-                    query_router.role(),
-                    self.process_id,
-                    round_robin,
-                )
+                .get(query_router.shard(), query_router.role(), self.process_id)
                 .await
             {
                 Ok(conn) => {
@@ -655,8 +649,6 @@ where
             let address = connection.1;
             let server = &mut *reference;
 
-            round_robin += 1;
-
             // Server is assigned to the client in case the client wants to
             // cancel a query later.
             server.claim(self.process_id, self.secret_key);