`Once` should return fail/complete status of decorated task, not `fail`

[eelstork]

Originally Once was implemented as a (thread safe) conditional. As such it returns:

• cont until the decorated task succeeds or fails (good)
• done or fail upon task completion (good)
• fail throughout later iterations, and until reset (bad)

In the third case, Once should retain and return the final status of the decorated task.

There isn't a solution here that's going to be compatible with thread pooling but, thread safety is on a best effort basis, not a requirement across the board.

Rarely used the Once decorator until recently. Seems a good alternative to ordered composites in a typical workflow, where stateless by default works well, and stateful nodes are ~1/10.

[eelstork]

c71bb04910e85575a6b2f98ac5fb3a2590808077