Fix Rust 1.37.0 compiler warnings (#5530)

Looks like most usages of trait objects should have introduced
a type variable instead.

client/src/rpc_client.rs

@@ -20,7 +20,7 @@ use std::thread::sleep;
 use std::time::{Duration, Instant};
 
 pub struct RpcClient {
-    client: Box<GenericRpcClientRequest + Send + Sync>,
+    client: Box<dyn GenericRpcClientRequest + Send + Sync>,
 }
 
 impl RpcClient {