Change node_events schema to use an id column as primary key rather than
node id because there can be multiple events per node id.
Change-Id: I518d8ef9ea658764876483e282a4058d3c4910f4
Add new table for node events. We can use this to notify node
operators of certain node events. Further, we can squash events for
multiple nodes with the same email into a single notification.
Change-Id: Icea6dd939df8fe4a98806bd79c014e21d239c43e