Checked config

The HeartbeatInterval is no longer directly configurable.

Instead the HeartbeatInterval is set to half the KeepAlive or a sixth of the DisconnectTimout if keep-alives are disabled.

The DefaultConfigurationManager ensures that the DisconnectTimeout and KeepAlive
values are functional.

This is an update to https://github.com/SignalR/SignalR/pull/1293, but it is targeted to the dev branch.

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