Remove dependency on rand's SliceRandom shuffle implementation in gossip-support (#2555)

In gossip-support, we shuffled the list of authorities using the
`rand::seq::SliceRandom::shuffle`. This function's behavior is
unspecified beyond being `O(n)` and could change in the future, leading
to network issues between nodes using different shuffling algorithms. In
practice, the implementation was a Fisher-Yates shuffle. This PR
replaces the call with a re-implementation of Fisher-Yates and adds a
test to ensure the behavior is the same between the two at the moment.