Create shell sessions through the async factory

ConsoleSession grew an async create and lost its constructor, and ShellSession
inherits that. sockapi was updated for the console branch but not the shell
branch immediately below it, so opening a shell session raised TypeError. It
is the only place in the tree that builds one.

confluent_server/confluent/sockapi.py

@@ -288,7 +288,7 @@ async def start_term(authname, cfm, connection, params, path, authdata, skipauth
             sessionid = elems[5]
         else:
             sessionid = None
-        consession = shellserver.ShellSession(
+        consession = await shellserver.ShellSession.create(
             node=node, configmanager=cfm, username=authname,
             datacallback=ccons.sendall, skipreplay=skipreplay,
             sessionid=sessionid, width=params.get('width', 80),