Support for gerrit stream events

[eddyseager]

Rather than SCM polling for gerrit, could the plugin exploit gerrit's stream events and trigger materials using go.cd's notifications API (https://api.go.cd/current/#notify-git-materials)

Here are the docs for gerrit stream events https://gerrit-documentation.storage.googleapis.com/Documentation/2.8.1/cmd-stream-events.html

There is already a Java library here which can be used to listen for such events. https://github.com/sonyxperiadev/gerrit-events. If the gerrit plugin used this library it could trigger the HTTP POST to notify the go.cd server of a material change rather than polling (which puts a lot of load on the server)