Server option object

[flowersinthesand]

To configure options, setter methods on server that correspond to each option have been adopted for the convenience of test but working with them in runtime on production may cause unpredictable behavior.

From that point of view, a dedicated object representing options fits better in this case.

[flowersinthesand]

Since the curren options apply to socket, a kind of socket configurer to be called in handshaking would be useful as a server option, i.e. options.socketConfigurer = (socket, headers) => {/* return configuration object including heartbeat for the given socket*/}

[flowersinthesand]

Follwing cettia/cettia-java-server#11 (comment), I will revert commits regarding this issue.