Babel: Seqno requests are properly decoupled from neighbors when the underlying inter...

Babel: Seqno requests are properly decoupled from neighbors when the underlying interface disappears

When an interface disappears, all the neighbors are freed as well. Seqno
requests were anyway not decoupled from them, leading to strange
segfaults. This fix adds a proper seqno request list inside neighbors to
make sure that no pointer to neighbor is kept after free.