Clarify assumptions that are being made

[joachimvh]

Feature description:

Certain assumptions are made in the code in certain places. E.g., some components assume slash semantics are valid, or even that the server is using LDP. It might be interesting to introduce something that checks for these classes that these are valid assumptions to make, and can warn if this is no longer the case.

We already have one situation where we do something similar: some classes assume that the server is single threaded and indicate this by implementing the SingleThreaded interface. The AppRunner then throws an error if multiple threads are detected but one of those components is loaded. This could potentially be generalized for more assumptions.