node: AsyncListener use separate storage mechanism

Before when an AsyncListener object was created and the "create"
callback returned a value, it was necessary to construct a new Object
with the same callbacks but add a place for the new storage value.

Now, instead, a separate storage array is kept on the context which is
used for any return value of the "create" callback. This significantly
reduces the number of Objects that need to be created.

Also added a flags property to the context to quickly check if a
specific callback was available either on the context or on the
AsyncListener instance itself.

Few other minor changes for readability that were difficult to separate
into their own commit.

This has not been optimized yet.