chore: only expose the method of setting random seed. (#45)

the user may be have set the random seed already.

lib/skywalking/util.lua

diff --git a/lib/skywalking/util.lua b/lib/skywalking/util.lua
index fe0548c..79580db 100644
--- a/lib/skywalking/util.lua
+++ b/lib/skywalking/util.lua
@@ -81,13 +81,15 @@
 end
 
 
-math.randomseed(random_seed())
+_M.set_randomseed = function ()
+    math.randomseed(random_seed())
+end
+
 
 local newID
 -- for Nginx Lua
 local ok, uuid = pcall(require, "resty.jit-uuid")
 if ok then
-    uuid.seed()
     newID = function()
         return uuid.generate_v4()
     end