Aggregator result equality easy to break

[nik9000]

Description

We'd like to make some optimizations that are gated behind the equals method on subclasses of InternalAggregation working perfectly. But at the moment it's quite easy implement equals incorrectly - our default equals implementation checks a few things but doesn't force subclasses to override it with their extra data. Many do, but not all. We should make not overriding equals here a compile time failure. Generally we do this by adding an abstract boolean doEquals method and calling it in equals. That's a bit tricky here because of all the layers of subclasses, but we can do it.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)