Distributor: Add possibility to run 'push' actions in background

Is your enhancement related to a problem? Please describe.

Currently the plug-in doesn't provide a way to run 'push' actions in background, which can be helpful in the cases when 'push' action has high resource consumption.

Describe the solution you'd like

Added hooks in the ajax_push(..) and send_notifications(..) functions to make possible to delay function logic execution and run in background.

Designs

Created a PR.

Describe alternatives you've considered

n/a

Additional context

n/a