Fix number of events stored in EventStore and adjust aggregation methods

[pitagoras3]

Before:

Earlier, each and every event occurring during migration was stored in EventStore. It was not problematic for small migrations, as most types of events are emitted only once per collection migration.

But there are two events - DumpUpdateEvent and RestoreUpdateEvent which are emitted every single time that mongodump/mongorestore writes on stdout. It occurs a lot of times (for long-taking migrations of large databases).

As number of events in EventStore grew up more and more, it was taking much more time to aggregate all the events, and calculate current state of migration.

After:

The most important change in this PR is the fact, that event of given type, can be stored only once in EventStore, and if there was earlier an event of a given type, it will be overriden.

fun storeEvent(event: StateEvent) {
    val eventsForCollection = events.getOrDefault(event.sourceToDestination, ConcurrentHashMap())
    eventsForCollection[event.type] = event
    events[event.sourceToDestination] = eventsForCollection
}

Thanks to that, number of events will be reduced to max of number_of_migrated_collections * types_of_events, which will significantly improve aggregation time.