make rpcClient compatible with 32bit arm systems (#2156)

On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to
arrange for 64-bit alignment of 64-bit words accessed
atomically. Only the first word in an allocated struct can
be relied upon to be 64-bit aligned.

client/rpc_client.go

@@ -21,10 +21,10 @@ import (
 )
 
 type rpcClient struct {
+	seq  uint64
 	once atomic.Value
 	opts Options
 	pool pool.Pool
-	seq  uint64
 }
 
 func newRpcClient(opt ...Option) Client {