Error when src-push to a remote: Message larger than max

Summary

When trying to push sources to a bst-artifact-server, the job fails with a grpc error.

Steps to reproduce

Relevant logs and/or screenshots

[00:00:24][a508b55d][src-push:stage0/freedesktop-junction.bst:bootstrap/linux-headers.bst] BUG     Src-push

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_scheduler/jobs/job.py", line 775, in child_action
        result = self.child_process()  # pylint: disable=assignment-from-no-return
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_scheduler/jobs/elementjob.py", line 105, in child_process
        return self._action_cb(self._element)
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_scheduler/queues/sourcepushqueue.py", line 44, in _push_or_skip
        if not element._source_push():
      File "/usr/local/lib/python3.7/dist-packages/buildstream/element.py", line 1904, in _source_push
        if not self.__sourcecache.push(source):
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_sourcecache.py", line 257, in push
        self.cas._send_directory(remote, source_proto.files)
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_cas/cascache.py", line 968, in _send_directory
        self.send_blobs(remote, missing_blobs)
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_cas/cascache.py", line 962, in send_blobs
        batch.send()
      File "/usr/local/lib/python3.7/dist-packages/buildstream/_cas/casremote.py", line 328, in send
        batch_response = local_cas.UploadMissingBlobs(self._request)
      File "/usr/local/lib/python3.7/dist-packages/grpc/_channel.py", line 565, in __call__
        return _end_unary_response_blocking(state, call, False, None)
      File "/usr/local/lib/python3.7/dist-packages/grpc/_channel.py", line 467, in _end_unary_response_blocking
        raise _Rendezvous(state, None, None, deadline)
    grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with:
        status = StatusCode.RESOURCE_EXHAUSTED
        details = "Received message larger than max (4660631 vs. 4194304)"
        debug_error_string = "{"created":"@1568022618.298376400","description":"Error received from peer unix:/tmp/buildstreamlzh5pivm/casd.sock","file":"src/core/lib/surface/call.cc","file_line":1052,"grpc_message":"Received message larger than max (4660631 vs. 4194304)","grpc_status":8}"