clippy cleanup #2

- remove needless return
- remove needless format!
- remove a few more needless clone()
- from_str_radix(_, 10) -> .parse()
- remove needless reference
- remove needless `mut`

Also manually replaced a match statement with map_err() because after
clippy was done with it, there was almost nothing left in the match
expression.

walkeeper/src/bin/wal_acceptor.rs

@@ -69,7 +69,7 @@ fn main() -> Result<()> {
 
     let mut conf = WalAcceptorConf {
         data_dir: PathBuf::from("./"),
-        systemid: systemid,
+        systemid,
         daemonize: false,
         no_sync: false,
         pageserver_addr: None,