Allow subscribing to events w/ or w/o first event - support-3.x

This PR is an equivalent to #482 but for support-3.x branch.

Emitting first event during the subscription process with the most recent value may not be always desired.

This is the case of the TangoAttributeEG class in Sardana, a subclass of the EventGenerator. It may work with the attribute which serves as a data pipe for transferring scan results. It is not desired to receive an event with the data corresponding to the previous scan while subscribing to events of the new scan.

Allow control whether to emit the first event during the subscription process.