[feature] Added retry mechanism with random backoff #186

[pandafy]

Added retry mechanism with random backoff for following functions:

• get_checksum
• download_configuration
• report_status

Closes #186

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I have manually tested these changes with my device.

[github-actions[bot]]

sh-checker report

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shellcheck errors

``` 'shellcheck ' returned error 1 finding the following syntactical issues: ---------- In openwisp_agent line 821: if [ "$?" -ne "0" ]; then ^--^ SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?. For more information: https://www.shellcheck.net/wiki/SC2181 -- Check exit code directly with e.g... ---------- You can address the above issues in one of three ways: 1. Manually correct the issue in the offending shell script; 2. Disable specific issues by adding the comment: # shellcheck disable=NNNN above the line that contains the issue, where NNNN is the error code; 3. Add '-e NNNN' to the SHELLCHECK_OPTS setting in your .yml action file. ```

shfmt errors

``` 'shfmt ' found no issues. ```