Reusing in-memory aggregate instances

[swistak35]

This is WIP.

• [ ] add the same argument to RailsEventStore::Client
• [ ] decide on the naming of after_store_callback

About the second point, options were enlisted here: https://github.com/RailsEventStore/rails_event_store/issues/290#issuecomment-505631048

There was no other suggestions than proposed by me in that comment, so:

1. yield
2. callback named argument (of course we can name it differently, if someone has better idea. I also thought that maybe we could name it _after_store_callback and NOT mention it in documention to sign that this is experimental/private feature which may change?)