Retain references to channel arguments

This works around issue 15662 which is not as easy to implement as I
would prefer it to be.

src/python/grpcio/grpc/_cython/_cygrpc/arguments.pyx.pxi

@@ -15,10 +15,12 @@
 cimport cpython
 
 
+# TODO(https://github.com/grpc/grpc/issues/15662): Reform this.
 cdef void* _copy_pointer(void* pointer):
   return pointer
 
 
+# TODO(https://github.com/grpc/grpc/issues/15662): Reform this.
 cdef void _destroy_pointer(void* pointer):
   pass
 

src/python/grpcio/grpc/_cython/_cygrpc/channel.pxd.pxi

@@ -69,3 +69,6 @@ cdef class Channel:
 
   cdef grpc_arg_pointer_vtable _vtable
   cdef _ChannelState _state
+
+  # TODO(https://github.com/grpc/grpc/issues/15662): Eliminate this.
+  cdef tuple _arguments

src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi

@@ -390,6 +390,7 @@ cdef class Channel:
   def __cinit__(
       self, bytes target, object arguments,
       ChannelCredentials channel_credentials):
+    arguments = () if arguments is None else tuple(arguments)
     grpc_init()
     self._state = _ChannelState()
     self._vtable.copy = &_copy_pointer
@@ -410,6 +411,7 @@ cdef class Channel:
         grpc_completion_queue_create_for_next(NULL))
     self._state.c_connectivity_completion_queue = (
         grpc_completion_queue_create_for_next(NULL))
+    self._arguments = arguments
 
   def target(self):
     cdef char *c_target

src/python/grpcio/grpc/_cython/_cygrpc/server.pxd.pxi

@@ -23,6 +23,7 @@ cdef class Server:
   cdef bint is_shutdown  # notification of complete shutdown received
   # used at dealloc when user forgets to shutdown
   cdef CompletionQueue backup_shutdown_queue
+  # TODO(https://github.com/grpc/grpc/issues/15662): Elide this.
   cdef list references
   cdef list registered_completion_queues