Allow soft-projections

[rosacris]

These kind of projections wont need to catchup/replay or to fetch missing events.
A possible use case is a price ticker where only the last update is useful.

[rosacris]

Another possibility is let the business logic to decide by including information with each domain event that indicates if it is catching up, replaying or on-line.

[andreapavoni]

Also useful to know/predict a state in the future.