Use Event.import instead of making N save calls

[mpraglowski]

This will lower number of SQL calls from N+2
to 3 (in best case) (where N is number of events).

[mostlyobvious]

For a brief moment I was worried about atomicity of adding events after these changes but there is a transaction block hidden inside add_to_stream.

[mostlyobvious]

I also realized that to_event_id.call(element) might have been a relict of database-generated uuids which we do not or want utilize in any way but maybe an author had this on thier mind.

[mostlyobvious]

Getting rid of to_event_id.call(element) first would have been much appreciated for review comfort ;)