Add ability to manage mediator pickup lifecycle manually

[TimoGlastra]

Currently you can initiate message pickup but it's not possible to stop the message pickup. When building a mobile wallet you often need more control over when the mediator pickup is active:

• when the device loses internet connection you may want to halt the pickup (other it will try to create a lot of ws connections but never succeed)
• when the device enters background mode you may want to halt the pickup
• when the device enters foreground mode again you may want to resume the pickup