job control: reuse common job code for rpc jobs

This makes stderr and exit callbacks work for rpc jobs

test/functional/api/rpc_fixture.lua

@@ -0,0 +1,38 @@
+local deps_prefix = './.deps/usr'
+if os.getenv('DEPS_PREFIX') then
+  deps_prefix = os.getenv('DEPS_PREFIX')
+end
+
+package.path = deps_prefix .. '/share/lua/5.1/?.lua;' ..
+               deps_prefix .. '/share/lua/5.1/?/init.lua;' ..
+               package.path
+
+package.cpath = deps_prefix .. '/lib/lua/5.1/?.so;' ..
+                package.cpath
+
+local mpack = require('mpack')
+local StdioStream = require('nvim.stdio_stream')
+local Session = require('nvim.session')
+
+local stdio_stream = StdioStream.open()
+local session = Session.new(stdio_stream)
+
+local function on_request(method, args)
+  if method == 'poll' then
+    return 'ok'
+  elseif method == 'write_stderr' then
+    io.stderr:write(args[1])
+    return "done!"
+  elseif method == "exit" then
+    session:stop()
+    return mpack.NIL
+  end
+end
+
+local function on_notification(event, args)
+  if event == 'ping' and #args == 0 then
+    session:notify("vim_eval", "rpcnotify(g:channel, 'pong')")
+  end
+end
+
+session:run(on_request, on_notification)