Introduce ProcessManager mixin similar to AggregateRoot in order to help with event sourced process managers

[mostlyobvious]

Possible implementation in: https://blog.arkency.com/process-managers-revisited/

[mostlyobvious]

Possibly related to #335

[tomaszwro]

after discussing with @pawelpacana: let's make something like https://github.com/arkency/aggregates, but for process managers

good example of a process manager is in https://github.com/RailsEventStore/cqrs-es-sample-with-res