Complete code refactoring

- `LOGGING` environment variable `json` or `human`
- `NODEID` environment variable (integer)
- Much cleaner go code
- Listener for exit of program to do cleanup
- All code is in packages except main.go
- Custom logger package added
- Connectivity checks reworked
- Healthcheck server on localhost only, so not exposed to outside world
- Updated `go.mod` and `go.sum`

docker-compose.yml

@@ -13,6 +13,8 @@ services:
       - DELAY=300
       - ROOTURL=
       - LISTENINGPORT=8000
+      - LOGGING=human
+      - NODEID=0
       - RECORD1=example.com,@,namecheap,provider,0e4512a9c45a4fe88313bcc2234bf547
       - RECORD2=example.info,@,namecheap,duckduckgo,157fd2a9c45a4fe88313bcc2234bfd58
       - RECORD3=example.io,www,namecheap,opendns,0e4512a9c45a4fe88313bcc2234bf547