Make threadpool::Runtime methods take &self (#1140)

The runtime is inherently multi-threaded, so it's going to have to deal
with synchronization when submitting new tasks anyway. This allows a
runtime to be shared by multiple threads more easily when e.g. building
a blocking facade over a tokio-based API.

tokio-tls/examples/download-rust-lang.rs

@@ -10,7 +10,7 @@ use tokio_io;
 use tokio_tls;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let mut runtime = Runtime::new()?;
+    let runtime = Runtime::new()?;
     let addr = "www.rust-lang.org:443"
         .to_socket_addrs()?
         .next()