Suspend http transactions for 30 seconds in case of failure

[matux]

Description of the change

We were doing this through the ConnectivityManager, which was removed yesterday.

This adds the suspension back with the main goal of keeping this behavior for the next release.

A proper solution will come when https://github.com/rollbar/rollbar-flutter/issues/70 is addressed.

A proper solution will see the payload flow abstracted frp-style into a stream of payloads over time and just model the flow of time declaratively and without managing state instead of this horribleness.

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [x] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Maintenance
• [ ] New release

Related issues

• Related to https://github.com/rollbar/rollbar-flutter/pull/69
• Related to https://github.com/rollbar/rollbar-flutter/issues/70

Checklists

Development

• [x] Lint rules pass locally
• [x] The code changed/added as part of this pull request has been covered with tests
• [x] All tests related to the changed code pass in development

Code review

• [x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [x] "Ready for review" label attached to the PR and reviewers assigned
• [x] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer