Fixed panic with signals

[Zhaars]

There is a panic with closing closed channels come from functions StopListeners and Exit. It happens because signals are processed asynchronously and corresponded function can be called several times which triggers the panic. As a solution was used sync.Once to make it callable only once.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation in the Acra documentation site or has PR on documentation repository
  with new changes
• CHANGELOG.md is updated (in case of notable or breaking changes)
• CHANGELOG_DEV.md is updated
• Benchmark results are attached (if applicable)
• Example projects and code samples are up-to-date (in case of API changes)

[Lagovas]

lgtm