Removed DetachThisObserver(). Instead, self-detaching observer functions can use a return value to request a detach. Detached observers process further events from the same turn.
Refactoring of engine mode names. X_queue -> X_concurrent.
Added explicit Continuations.

Before, all input in observer nodes to reactives of the current domain was captured as a continuation. Input to other domains was not allowed.
Now, MakeContinuation can be used to create explicit continuations. Each continuation is processed as a transaction. Continuations to other domains are supported.