Increase hackney_sup supervisor restart tolerance

Generally found this to be a good idea for top level app supervisors,
especially as part of proper releases.
fix hackney_manager child spec: supervisor -> worker
Make hackney_pool ets from hackney_manager

Making it from the supervisor is unnecessary. Trying to create a new named
ets table that already exists will crash, like when supervisor:init is called
again during release upgrades, to check if the child specs have changed.
add a start-dev script, for convenience