Rename first adapter in the pool for backwards compatability (#169)

Pubsub v2.0 expects there to be a single process for the process group
named `Elixir.my_pubsub.Adapter`.

v2.1 added pools, which means that the processes are supervised with a
supervisor called `Elixir.my_pubsub.Adapter` along with n processes
named `Elixir.my_pubsub.Adapter_n` where n is the pool size.

This commit changes the supervisor name to
`Elixir.my_pubsub.Adapter_supervisor`
and renames pool item 1 to be `Elixir.my_pubsub.Adapter`. This will
allow backwards compatability with v2.0 when the pool size is set to 1.

lib/phoenix/pubsub/pg2.ex

@@ -62,16 +62,20 @@ defmodule Phoenix.PubSub.PG2 do
     name = Keyword.fetch!(opts, :name)
     pool_size = Keyword.get(opts, :pool_size, 1)
     adapter_name = Keyword.fetch!(opts, :adapter_name)
-    Supervisor.start_link(__MODULE__, {name, adapter_name, pool_size}, name: adapter_name)
+    Supervisor.start_link(__MODULE__, {name, adapter_name, pool_size}, name: :"#{adapter_name}_supervisor")
   end
 
   @impl true
   def init({name, adapter_name, pool_size}) do
-    groups =
+    [_ | groups] =
       for number <- 1..pool_size do
         :"#{adapter_name}_#{number}"
       end
 
+    # Use `adapter_name` for the first in the pool for backwards compatability
+    # with v2.0 when the pool_size is 1.
+    groups = [adapter_name | groups]
+
     :persistent_term.put(adapter_name, List.to_tuple(groups))
 
     children =