…rformance (#2246)

Original PR: #2159

## Breaking changes

- `[p2p]` Rename `IPeerSet#List` to `Copy`, add `Random`, `ForEach`
methods.
   Rename `PeerSet#List` to `Copy`, add `Random`, `ForEach` methods.

Fixes #2158

## Performance improvement

This change makes PeerSet.Remove much more efficient simply by using
more idiomatic Go re-slicing to avoid the prior mechanisms of creating
fresh peer set lists on just a single remove. While here also added a
remedy for a found bug #2158 due to an abstraction that returns a stale
slice to its caller in Switch.OnStop.

Benchmark results:

```shell
$ benchstat before.txt after.txt
name                 old time/op    new time/op    delta
PeerSetRemoveOne-8     90.5µs ± 4%    95.9µs ±13%     ~     (p=0.218 n=10+10)
PeerSetRemoveMany-8    1.58ms ± 4%    1.50ms ± 1%   -4.98%  (p=0.000 n=10+8)

name                 old alloc/op   new alloc/op   delta
PeerSetRemoveOne-8     8.48kB ± 0%    7.92kB ± 0%   -6.60%  (p=0.000 n=10+10)
PeerSetRemoveMany-8     149kB ± 0%      65kB ± 0%  -56.44%  (p=0.000 n=10+10)

name                 old allocs/op  new allocs/op  delta
PeerSetRemoveOne-8       85.0 ± 0%      73.0 ± 0%  -14.12%  (p=0.000 n=10+10)
PeerSetRemoveMany-8     1.32k ± 0%     1.22k ± 0%   -7.51%  (p=0.000 n=10+10)
```

which savings become so much more when peers are removed much more
frequently for a longer period of time and could mitigate DOS vectors.

Fixes #2157

---------

Co-authored-by: Emmanuel T Odeke <emmanuel@orijtech.com>
Co-authored-by: Sergio Mena <sergio@informal.systems>
(cherry picked from commit e1ee71c)