kcqrs/eventstore: load multiple aggregates by single key

[mgenov]

It would be valuable if EventStore is able to load multiple aggregates by providing single key.

For example the API will look like

// Returns all events of all aggregates related to the provided key.
// The result may contain: License, Location, Addreess and etc aggreegates
val events = eventStore.getEventsByKey("Customer:123") 

// Returns all Licenses attached to the provided customer
val licenses = eventStore.getAggregatesByKey("Customer:123", LicenseAggregate.key);

For small collections it will ensure that data is consistent and will skip view creation at some places.

Here is a prototype of the message definition which could be persisted in the datastore:

message RowEvents {
  
  string rowId = 1;

  repeated AggregateEvent events = 2;
}

message AggregateEvent {

  string aggregateId = 1;

  string aggregateType = 2;

  int64 timestamp = 3;

  string identityId = 4;
  
  bytes payload = 5;
}