nib: some fixes when a router or a prefix is deleted

[fabian18]

Contribution description

I still had some fixes for NIB lying around. I remember they were necessary during my thesis. The fixes are old and probably would need some confirmation, but I think they are still valid.

Testing procedure

I will try to show their necessity by adding something to our tests.

Issues/PRs references

[riot-ci]

Murdock results

:heavy_check_mark: PASSED

bb9ca2ef8661124ed66c1a4d8a89235ceb9b22dd tests/net/gnrc_ipv6_nib: add tests for router and prefix timeout

Success	Failures	Total	Runtime
10006	0	10008	07m:34s

Artifacts

[benpicco]

Are all the changes from this PR now merged or is there still something left?

[fabian18]

Not all, I am about to rebase this PR

[benpicco]

@miri64 have your comments been addressed?

[miri64]

I will have a look at this, and the other GNRC PRs, next week at Hack'n'ACK.