THRIFT-2883 Fix dict changed size during iteration exception

During the notification of outstanding requests via errback that the
  connection has failed, new requests can be attempted which change
  the size of the client._reqs dict.  This change uses a temp variable
  to hold the old dict, and resets client._reqs to a new, empty dict.
  Also, it iterates until the new dict does not have any requests added
  by the errbacks.