Refactor Nomad Recovery

from an approach that loaded the runners only once at the startup
to a method that will be repeated i.e. if the Nomad Event Stream connection interrupts.

internal/runner/abstract_manager.go

@@ -123,5 +123,3 @@ func (n *AbstractManager) Get(runnerID string) (Runner, error) {
 func (n *AbstractManager) Return(_ Runner) error {
 	return nil
 }
-
-func (n *AbstractManager) Load() {}