cbbackup/cbtranster: Don't handle doc with very large rev

Traceback (most recent call last):

  File "/opt/couchbase/lib/python/runtime/lib/python3.7/threading.py", line 917, in _bootstrap_inner

    self.run()

  File "/opt/couchbase/lib/python/runtime/lib/python3.7/threading.py", line 865, in run

    self._target(*self._args, **self._kwargs)

  File "/opt/couchbase/lib/python/pump.py", line 446, in run_worker

    curx).run()

  File "/opt/couchbase/lib/python/pump.py", line 504, in run

    rv_batch, batch = self.source.provide_batch()

  File "/opt/couchbase/lib/python/pump_dcp.py", line 254, in provide_batch

    rv, batch = self.provide_dcp_batch_actual()

  File "/opt/couchbase/lib/python/pump_dcp.py", line 413, in provide_dcp_batch_actual

    msg = (cmd, vbucket_id, key, flg, exp, cas, rev_seqno.to_bytes(4, 'big'), val, seqno, dtype, metalen,

OverflowError: int too big to convert