Implement TCP keepalive and/or the RFB KeepAlive message

[dcommander]

Referring to #378, there are use cases in which these features would be beneficial, but it would take some effort to determine the scope of those use cases and to implement, test, and document the features. This effort is best conducted in the context of a funded development contract with an organization that has a need for such features and can test them on a large scale.