[blockchain] Upgrade tokio

- Also upgrade reqwest
- Switch to `tokio::runtime::Builder::new_single_thread()` because
`tokio::runtime::Runtime::new()` changed it's behavior to create a
multithreaded runtime.
- `enable_all` enables time and io resource drivers as explained
[here](https://docs.rs/tokio/0.2.24/tokio/runtime/index.html#resource-drivers)

macros/src/lib.rs

@@ -145,7 +145,7 @@ pub fn await_or_block(expr: TokenStream) -> TokenStream {
         {
             #[cfg(all(not(target_arch = "wasm32"), not(feature = "async-interface")))]
             {
-                tokio::runtime::Runtime::new().unwrap().block_on(#expr)
+                tokio::runtime::Builder::new_current_thread().enable_all().build().unwrap().block_on(#expr)
             }
 
             #[cfg(any(target_arch = "wasm32", feature = "async-interface"))]