Let AggregateStore use 'ChangeSets' rather than separate Commands and Events

[timbru]

Currently we save a Command key, and then 0 (in case of an error) or more Events (in case there was a change).

We can simplify this by saving a single ChangeSet structure that includes all this. Just like WalStore does.

This will simplify the logic around finding these files. Each change will be in a single atomic stored key. It will also reduce the number of keys (files for most deployments).

[timbru]

We will also need to re-work command history when we do this:

• Update StoredCommand:
  • Update StoredEffect to include full events
  • Use as the single thing to save in AggregateStore
  • Use as the single thing to apply in Aggregate
  • Fix recover function to archive corrupt StoredCommands
  • Use predictable keys for processed commands in AggregateStore (sequence)
  • Remove sequence from Command (Command version is used)
  • Remove version from Event (Command version is used)
  • Remove info file (no longer needed with predictable command keys)
  • Use command and event (specific variant) for init
  • Get init event from command 0 for get_aggregate without snapshots.
• Support migrations
  • Use OldStoredCommand for migrations (it includes event nrs not events)
  • Pre_0_14_0
    • Parse all commands and events in old,
    • convert to new style StoredComment
    • save in new
    • ignore the info (won't be needed with new predictable key names)
  • Pre_0_10_0
    • Same as above.. convert old command
• Keep history structure now that command keys are simple
  • Add optional history_cache to AggregateStore
  • Add disable_history_cache option (save memory if needed)
  • Fill cache on warmup
  • Add new command to cache
  • Show history records from cache / key value store