Subscriptions pool #192

ESDB has this smart functionality that allows maintaining less subscriptions if all the event handlers are live. In Eventuous context it would work like this:

• Each subscription (group of handlers) is independent and can start from its own checkpoint
• When subscription groups are catching up, they keep running independently
• When a group starts to run live it switches to a single catch-up subscription, which runs live. The catch-up subscription used by the group gets disposed.
• When all the groups run live, there's only one physical catch-up subscription running
• If a particular group starts lagging behind, it switches to its own catch-up subscription

As a result, when all the groups are running live, there's only one physical catch-up subscription to the database, so there's less traffic and less read pressure on the database.

_{From SyncLinear.com | EVE-38}