chore(cache) set shm_set_retries option to 3

This is the default value offered by mlcache, but set here for
the sake of explicitness.

Author: thibaultcha

kong/cache.lua

@@ -71,6 +71,7 @@ function _M.new(opts)
 
   local mlcache, err = resty_mlcache.new(SHM_CACHE, SHM_CACHE, {
     shm_miss         = "kong_db_cache_miss",
+    shm_set_retries  = 3,
     lru_size         = LRU_SIZE,
     ttl              = max(opts.ttl     or 3600, 0),
     neg_ttl          = max(opts.neg_ttl or 300,  0),