fix(cluster) use 'safe_set' to set prepared query id

We do not want to unintentionally override some cluster info and would
rather fail to cache the query id in the shm instead. Other workers will
fail to retrieve the key from it and will thus prepare the request
themselves, so we maintain a working state.

lib/resty/cassandra/cluster.lua

@@ -611,8 +611,15 @@ local function get_or_prepare(self, coordinator, query)
         query_id, err = prepare(self, coordinator, query)
         if not query_id then return nil, err end
 
-        local ok, err = shm:set(key, query_id)
-        if not ok then return nil, 'could not set query id in shm: '..err end
+        local ok, err = shm:safe_set(key, query_id)
+        if not ok then
+          if err == 'no memory' then
+            log(WARN, _log_prefix, 'could not set query id in shm: ',
+                      'running out of memory, please increase the ',
+                      self.dict_name, ' dict size')
+          else
+            return nil, 'could not set query id in shm: '..err end
+          end
       end
 
       local ok, err = lock:unlock()