Fetch data example frequently panics after root lookup

[cpswan]

Describe the bug

RUST_LOG=info cargo run --example fetch_data_example ~/.atsign/keys/@cpswan_key.atKeys cpswan bareindoornetball
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
     Running `target/debug/examples/fetch_data_example '/home/chris/.atsign/keys/@cpswan_key.atKeys' cpswan bareindoornetball`
[2023-06-20T11:34:42Z INFO  at_rust::at_secrets] Extracting keys from file
[2023-06-20T11:34:42Z INFO  at_rust::at_secrets] Decoding keys
[2023-06-20T11:34:42Z INFO  at_rust::at_secrets] Keys decoded and decrypted
[2023-06-20T11:34:42Z INFO  at_rust::at_client] Getting at sign server address
[2023-06-20T11:34:43Z INFO  at_rust::at_tls_client] Connected to AtServerAddr { host: "root.atsign.org", port: 64 }
[2023-06-20T11:34:43Z INFO  at_rust::at_tls_client] Sending: cpswan
[2023-06-20T11:34:44Z INFO  at_rust::at_tls_client] Received: @
[2023-06-20T11:34:44Z INFO  at_rust::at_tls_client] Closing connection
[2023-06-20T11:34:44Z INFO  at_rust::at_client] At server address:
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', src/at_client.rs:188:16
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Steps to reproduce

1. First I run RUST_LOG=info cargo run --example fetch_data_example ~/.atsign/keys/@cpswan_key.atKeys cpswan bareindoornetball
2. Then I see the error message above quite often

Expected behavior

Client should be more resilient, and retry with exponential backoff until a configurable number of attempts has failed.