Create a Listener/WaitSet functionaliy

[elBoberido]

Currently it's not possible to wait for multiple subscriber in the same thread since there is one condition_variable used for each Subscriber. In order to wait for multiple subscriber in one thread, a similar functionality like the Listener/WaitSet from the C++ implementation is needed.

Due to the ownership semantics something similar to a stateless WaitSet might be the best solution for this problem. The user would need to start a thread by oneself, which is also more idiomatic rust than starting a thread in the background.

If this feature is needed in the short therm, the corresponding C++ classes could be wrapped.

[elBoberido]

@mossmaurice fyi since you asked for this :)