etcdserver: move int64Slice into pkg/types/

pkg/types/slice.go

@@ -0,0 +1,8 @@
+package types
+
+// Int64Slice implements sort interface
+type Int64Slice []int64
+
+func (p Int64Slice) Len() int           { return len(p) }
+func (p Int64Slice) Less(i, j int) bool { return p[i] < p[j] }
+func (p Int64Slice) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }