Ordered single-threaded executor

[jacquelinekay]

Suggestion from @adolfo-rt:

Currently the ordering of tasks in the single-threaded executor is not exposed to the user. It should be possible to implement a single-threaded executor in which task execution ordering can be specified by the user. This is to further decouple the component topology from its thread execution.

Not a high priority for the current milestone (ROSCon demos) but an eventual goal.