Add wtfnode api endpoint

[dapplion]

Run wtfnode on demand through a dedicated API route.

I've added this to debug the socket open handles leak but I think it's a very valuable tool to keep mid-term. wtfnode is loaded with a lazy require call so it will only be loaded after the first call to the API route. Otherwise won't be run at all and don't disturb non-debug runs

Sample response

Replies with a plain text formated dump

http://localhost:9596/eth/v1/lodestar/wtfnode/

[WTF Node?] open handles:
- File descriptors: (note: stdio always exists)
  - fd 1 (tty) (stdio)
  - fd 2 (tty) (stdio)
  - fd 0 (tty)
- Sockets:
  - 192.168.1.36:45442 -> 91.105.22.151:9000
  - 192.168.1.36:50264 -> 188.166.75.68:13000
  - 192.168.1.36:50558 -> 69.230.152.160:9000
  - 192.168.1.36:49432 -> 85.5.238.105:13000
  - 192.168.1.36:49006 -> 71.47.209.223:13000
  - 192.168.1.36:45112 -> 93.56.200.143:13000
  - 192.168.1.36:41760 -> 86.20.4.83:9000
  - 192.168.1.36:50024 -> 46.39.104.43:9000
  - 192.168.1.36:43430 -> undefined:undefined
    - Listeners:
      - connect: onConnect @ /home/lion/Code/eth2.0/lodestar/node_modules/libp2p-tcp/src/index.js:101
  - 192.168.1.36:55328 -> undefined:undefined
    - Listeners:
      - connect: onConnect @ /home/lion/Code/eth2.0/lodestar/node_modules/libp2p-tcp/src/index.js:101

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