ForwardFromChannels - reads messages from all input channels, and calls the `onMessage` callback.

HandleUnsubscribe - when the client calls "unsubscribe" or the subscription times out, it calls `onSub` Must be called as a go routine!.

HandleUnsubscribeErrChan - when the client calls "unsubscribe" or the subscription times out, it calls `onSub` Must be called as a go routine!.

NewManager creates a new subscription manager.

connect - function that returns the input channel.

Manager is a subscription manager - allows for thread-safe registering/unregistering subscribers of type T.

NewHeadsService multiplexes new batch header messages from an input channel into multiple subscribers also handles unsubscribe Note: this is a service which must be Started and Stopped.