[consensus] Fix heartbeat batching upper limit.

multi_raft_heartbeat_window_ms should be no more than
raft_heartbeat_interval_ms / 2.
Currently it is capped at FLAGS_consensus_rpc_timeout_ms / 2,
which is simply wrong.

Change-Id: Ib52f21238c7c9e76828edf5582f12b063234f56b
Reviewed-on: http://gerrit.cloudera.org:8080/23952
Tested-by: Kudu Jenkins
Reviewed-by: Zoltan Chovan <zchovan@cloudera.com>
Reviewed-by: Gabriella Lotz <lotzgabriella@gmail.com>
Reviewed-by: Alexey Serbin <alexey@apache.org>