Thread startup shouldn't block on Actor<Object> construction

[jfirebaugh]

The Thread constructor blocks until the spawned thread has constructed the object. For DefaultFileSource this includes creating the offline database, which could be a lengthy operation.

We should modify Thread so that it doesn't block. This will be tricky, requiring the ability to construct an ActorRef<Object> for an Object that might not have itself been constructed yet.

[kkaefer]

Instead of changing our Actor system, we could make the OfflineDatabase construction faster by not opening the database in the constructor, and instead open it in a self-sent message as the first message processed for this object.