Graceful shutdown on stop()

[marci4]

Description

Added a graceful shutdown during stop()
This should make sure, that all remaining frames are send to the other endpoints

Related Issue

Fixes #620

Motivation and Context

Interrupting the selector thread is not the right way to cause the shutdown

How Has This Been Tested?

Tested with JUnit test Issue608

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.