clist: speedup Next by removing defers (#2511)

This change doubles the speed of the mempool's reaping.
Before:

BenchmarkReap-8   	    5000	    365390 ns/op	  122887 B/op

After:

BenchmarkReap-8   	   10000	    158274 ns/op	  122882 B/op

libs/clist/clist.go

@@ -113,9 +113,9 @@ func (e *CElement) NextWaitChan() <-chan struct{} {
 // Nonblocking, may return nil if at the end.
 func (e *CElement) Next() *CElement {
 	e.mtx.RLock()
-	defer e.mtx.RUnlock()
-
-	return e.next
+	val := e.next
+	e.mtx.RUnlock()
+	return val
 }
 
 // Nonblocking, may return nil if at the end.