Pool IDB connections to reduce main thread activity

[TimvdLippe]

We are currently opening a new IDB connection for every actor. Even with a couple of actors, this generates quite a strain on the main thread.

Rather than having a connection per actor, we should have 1 connection per realm. We already create 1 store per realm, so the fix would be to create a list of actors that are interested in updates. Then we debounce the actual IDB connection, iterate through the rows and then call each interested actor. This should significantly reduce the amount of time spent waiting on IDB.