Refactor Shared Memory Object Creation

## Brief feature description

Both `PoshRuntime` and `ProcessManager` don't follow our main design principles (RAII, clear responsibilty, etc.). Hence, a refactoring is necessary.

## Detailed information

- [x] Rename `ProcessName_t` into `PoshRuntimeName_t`
- [x] Remove `RouDiLock`
- [ ] Create design document
- [ ] Rename `ProcessManager` to `RuntimeManager`?
- [ ] Remove `IOX_DISCARD_RESULT` around `registerProcess/unregisterProcess` in roudi.cpp and inform user (comes from #637)
- [ ] Agree on design
- [ ] Implement design
- [ ] Use `reference_wrapper` for `SegmentUserInformation.m_memoryManager`, [see](https://github.com/eclipse-iceoryx/iceoryx/pull/598#discussion_r599669633)
- [ ] remove the `cxx::not_null` from `cxx::not_null` in `ProcessManger::addProcess` since an application without write access to a data segment would crash RouDi ( have a look at `SegmentManager::getSegmentInformationForUser`)

Start with the checklist and design discussion, then inspect roudi.cpp, ProcessManager::addProcess, SegmentUserInformation.m_memoryManager, and SegmentManager::getSegmentInformationForUser. Establish the design agreement before implementation, including the RuntimeManager and error-handling questions. Done means the agreed design is documented and all remaining refactoring items are implemented and validated.

Written by the indexing model from the issue text.