NMPlatform's cache should be directly accessible to the users,
at least the NMPLookup part and the fact that the cache contains
ref-counted, immutable NMPObjects.

This allows users to inspect the cache with zero overhead. Meaning,
they can obtain an NMDedupMultiHeadEntry and iterate the objects
themself. It also means, the are free to take and keep references
of the NMPObject instances (of course, without modifying them!).

NMFakePlatform will use the very same cache. The fake platform should
only differ when modifying the objects.

Another reason why this makes sense is because NMFakePlatform is for one
a test-stup but also tests behavior of platform itself. Using a separate
internal implementation for the caching is a pointless excecise, because
only the real NMPCache's implementation really matters for production.
So, either NMFakePlatform behaves idential, or it is buggy. Reuse it.

Port fake platform's tracking of routes to NMPCache and move duplicate
code from NMLinuxPlatform to the base class.

This commit only ports IP routes, eventually also addresses and links
should be tracked via the NMPCache instance.