Add full integration test/simulation of messages from multiple nodes being processed

[clintharris]

• verify that event ordering is correct (i.e., that various table/row/column values end up with values specified by "most recent" corresponding operation)
• this will require implementation of in-memory data store
• how will this work without the use of the merkle tree for efficient diffing? is that even necessary?
• make sure to prevent the "many changes in same ~10sec window causes issue" problem (https://twitter.com/jaredforsyth/status/1228366315569565696)