implement the operations API

[the-mikedavis]

connects #7

there's some cool stuff in here we can do especially with scavenges like controlling the thread count dedicated to performing the scavenge

also the scavenge workflow is asynchronous, so you can do something like

iex> {:ok, scavenge} = Spear.start_scavenge(conn)
iex> {:ok, sub} = Spear.subscribe(conn, self(), Spear.scavenge_stream(scavenge))
iex> receive do
...>   %Spear.Event{type: "$scavengeCompleted"} -> :done
...> after
...>   60_000 -> :timeout
...> end
:done
iex> Spear.cancel_subscription(conn, sub)
:ok

to await the scavenge completion

other stuff doesn't seem to have any utility yet like node resignation or priorities. those will probably come into play when we do some clustering & gossip