Add tests for stats processing

[wojcik91]

Implement tests with mock gateways to verify that peer stats are processed as expected.
Things to test:

• new sessions are created once a first stats update comes
• if a session already exists a duplicate is not created
• if a session with state new already exists it's changed to connected once stats are received
• peer updates for invalid keys are discarded
  - [ ] peer updates without an endpoint are discarded (this is done in the gateway handler)
• peer updates in an invalid order are discarded
• changing a device public key is handled properly
• upload & download calculations work as expected
• sessions are disconnected once enough time passes since last timestamp

Result: Integration tests for the session manager run in the CI (and can be run manually)