add retry to slack client when "migration_in_progress" error returned(https://github.com/bcicen/slacksocket/issues/19)
add retry to slack client when "migration_in_progress" error
returned(https://github.com/bcicen/slacksocket/issues/19)

vektorlab/slacksocket,graywizardx/slacksocket,bcicen/slacksocket