protocols.pc_rpc: exclude kernel_invariants from proxying.

Fixes #531.

artiq/protocols/pc_rpc.py

@@ -100,6 +100,8 @@ class Client:
         in the middle of a RPC can break subsequent RPCs (from the same
         client).
     """
+    kernel_invariants = set()
+
     def __init__(self, host, port, target_name=AutoTarget, timeout=None):
         self.__socket = socket.create_connection((host, port), timeout)
 
@@ -187,6 +189,8 @@ class AsyncioClient:
     Concurrent access from different asyncio tasks is supported; all calls
     use a single lock.
     """
+    kernel_invariants = set()
+
     def __init__(self):
         self.__lock = asyncio.Lock()
         self.__reader = None
@@ -286,6 +290,8 @@ class BestEffortClient:
     :param retry: Amount of time to wait between retries when reconnecting
         in the background.
     """
+    kernel_invariants = set()
+
     def __init__(self, host, port, target_name,
                  firstcon_timeout=0.5, retry=5.0):
         self.__host = host