Enable restart_kernel for async usage

Converted `MappingKernelManager.restart_kernel` to a coroutine so that
projects that take advantage of async kernel startup can also realize
appropriate behavior relative to restarts.

Author: kevin-bates

jupyter_server/services/kernels/kernelmanager.py

@@ -306,10 +306,11 @@ def shutdown_kernel(self, kernel_id, now=False):
 
         return super(MappingKernelManager, self).shutdown_kernel(kernel_id, now=now)
 
+    @gen.coroutine
     def restart_kernel(self, kernel_id):
         """Restart a kernel by kernel_id"""
         self._check_kernel_id(kernel_id)
-        super(MappingKernelManager, self).restart_kernel(kernel_id)
+        yield gen.maybe_future(super(MappingKernelManager, self).restart_kernel(kernel_id))
         kernel = self.get_kernel(kernel_id)
         # return a Future that will resolve when the kernel has successfully restarted
         channel = kernel.connect_shell()
@@ -345,7 +346,7 @@ def on_restart_failed():
         channel.on_recv(on_reply)
         loop = IOLoop.current()
         timeout = loop.add_timeout(loop.time() + self.kernel_info_timeout, on_timeout)
-        return future
+        raise gen.Return(future)
 
     def notify_connect(self, kernel_id):
         """Notice a new connection to a kernel"""