[Feature Request] Set different upstream servers for every listening address

Usually, I set 8.8.8.8 and 8.8.4.4 as main DNS servers. When connections are unstable, it will switch to backup DNS servers.
```
./dnsproxy -l 127.0.0.1 -p 53 -u 8.8.8.8:53 -u 8.8.4.4:53
./dnsproxy -l 127.0.0.2 -p 53 -u 1.1.1.1:53 -u 1.0.0.1:53
```
Like the above, I have to start ```dnsproxy``` twice. I hope in the future we can specify upstream servers for each listening address in one command line.

Start at the dnsproxy command-line entry point and trace how repeated -l and -u options are parsed and how listening addresses receive upstreams. Confirm the intended configuration from the two command examples; done means one invocation can assign distinct upstream server sets to each listening address while preserving failover behavior.

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