Reformat imports to a consistent style for imports

rustfmt.toml configuration:
        imports_granularity = "One"
         group_imports = "One"

core/src/sample_performance_service.rs

@@ -1,12 +1,14 @@
-use solana_ledger::{blockstore::Blockstore, blockstore_meta::PerfSample};
-use solana_runtime::bank_forks::BankForks;
-use std::{
-    sync::{
-        atomic::{AtomicBool, Ordering},
-        Arc, RwLock,
+use {
+    solana_ledger::{blockstore::Blockstore, blockstore_meta::PerfSample},
+    solana_runtime::bank_forks::BankForks,
+    std::{
+        sync::{
+            atomic::{AtomicBool, Ordering},
+            Arc, RwLock,
+        },
+        thread::{self, sleep, Builder, JoinHandle},
+        time::{Duration, Instant},
     },
-    thread::{self, sleep, Builder, JoinHandle},
-    time::{Duration, Instant},
 };
 
 const SAMPLE_INTERVAL: u64 = 60;