notification agent coordination

Currently, the notification agent can function in active/active HA where each
notification agent grabs notifications from the message queue in a FIFO type
rotation (it checks for message(s), grabs message(s)). The issue that arises is
that if we want to implement a pipeline to process events, we cannot guarantee
what event each agent worker will get and because of that, we cannot enable
transformers which aggregate/collate some relationship across similar events.

This also fixes an existing bug where if multiple notification agents are
enabled, the pipeline transformers may not work as expected because it will
only act on the samples it sees on current worker.