Verify that `PointerRepository` does not have to be thread-safe

[elfenpiff]

Brief feature description

In a this discussion it was unraveled that the PointerRepository is not-thread-safe: https://github.com/eclipse-iceoryx/iceoryx/pull/1699#discussion_r984338244

@elBoberido drafted the following possible scenario:

• thread A is started
• thread B is started
• thread A registers a pointer
• thread B registers a pointer -> is this a valid use case
• thread B uses a relative pointer -> do we need to sync the memory

Do we require memory synchronizations here or even place some kind of thread-safe mechanism? This has to be investigated further.

In my opinion the drafted scenario is a valid use case but I think that iceoryx does not use the pointer repository in that fashion. A startup looks like:

• runtime registers at roudi
• roudi sends a list with all shared memory segments to the application
• one thread in the application receive this list and registers all segments in the pointer repository
• since the runtime init is the first call threads will start to pop up after the registration is finalized

Furthermore, the pointer repository is an internal construct. So I think that we currently do not have a bug but since the expected behavior differs from the actual one we require a solution for the scenario drafted by @elBoberido . Which includes

• threadsafe pointer registration
• sync between threads

Or we try to avoid this by defining a strict contract. The problem is unregister, what happens when a thread unregisters a segment while others are still using them (or the relative pointers on them).