Refactor libp2p::Peer

[gumb0]

I don't remember exactly where I went with this, but my best guess is that I wanted to achieve the following:

• Remove the ability to change Peer's endpoint. When endpoint changes, new Peer instance should be created.
• Get rid of inheritance Peer->Node, make Node a member of Peer instead.
• Get rid of friends of Peer (RLPXHandshake looks already not required to be a friend)

[codecov-io]

Codecov Report

Merging #5265 into master will increase coverage by 0.03%.
The diff coverage is 72.5%.

@@            Coverage Diff             @@
##           master    #5265      +/-   ##
==========================================
+ Coverage   62.33%   62.37%   +0.03%     
==========================================
  Files         350      350              
  Lines       29428    29427       -1     
  Branches     3322     3321       -1     
==========================================
+ Hits        18344    18354      +10     
+ Misses       9886     9878       -8     
+ Partials     1198     1195       -3

[gumb0]

Rebased.