Remove UDP network protocol

[thsfs]

Most of the related source-code to UDP network protocol is now dead code. UDP should be removed.

• [x] Rewrite unit tests from UDP to TCP or loopback channels.
  • [x] Review the UDP tests that uses add_outer_node ()to see if they can be written using the inproc channel.
• [ ] Remove everything UDP related.
  • [x] Remove UDP implementation.
  • [ ] Remove no longer needed config options.
  • [x] Remove related stats
  • [x] Check there is no related unused function and structures.

[thsfs]

Update also upon the changes from this PR: https://github.com/nanocurrency/nano-node/pull/3851