Server should not be up if too many stores cannot start up

[pnarayanan]

Set a threshold for the acceptable number of down stores for a server to even come up. Due to configuration issues or due to multiple disk failures, it is possible that too many stores are unable to start up. In such cases, it is probably wise to not let the server start up. Today, a server can start up even when none of the stores do, which may result in these servers receiving requests they cannot process.