Use single read and write buffer where possible

cluster.go

@@ -8,6 +8,7 @@ import (
 	"math"
 	"math/rand"
 	"net"
+	"runtime"
 	"sort"
 	"sync"
 	"sync/atomic"
@@ -82,6 +83,10 @@ func (opt *ClusterOptions) init() {
 		opt.ReadOnly = true
 	}
 
+	if opt.PoolSize == 0 {
+		opt.PoolSize = 5 * runtime.NumCPU()
+	}
+
 	switch opt.ReadTimeout {
 	case -1:
 		opt.ReadTimeout = 0