In Class ServerManager, onlineServers is declared as ConcurrentHashMap. In findServerWithSameHostnamePortWithLock(), it has to do a loop to find if there is a ServerName with same host:port pair. If replaced with ConcurrentSkipListMap, findServerWithSameHostnamePortWithLock() can be replaced with a O(logN) implementation.
I run some performance comparison(test the function only), it seems that there is no difference if there are 1000 servers. With more servers, ConcurrentSkipListMap implementation is going to win big.