improve readability of member event coalescer

[mbver]

• lastEvents and latestEvents are easily confused with one another. recommend using newEvents. coalesceEvent is not descriptive of its role. it is event for a node, so call it nodeEvent.
• using only event types for lastEvents is too clever and make the code harder to reason about. furthermore, it doesn't help in checking duplicating member-update-event. stick with nodeEvent for lastEvents and it handles duplication with ease.