Added cluster_version_t::CLUSTER.  The idea is to replace some uses of ONLY_VERSION with CLUSTER.
Made rdb_protocol/batching.hpp use CLUSTER instead of ONLY_VERSION.
Removed ONLY_VERSION assertions in versioned.hpp, used v1_13_is_latest where appropriate.
Replaced ONLY_VERSION with CLUSTER in generate_rpc_templates.py, made the assertion appear in all arities.
Used serialize_universal in connectivity/multiplexer.cc, also added static assertions about is_same<tag_t, uint8_t>.
Made cluster.cc more appropriate, made it use cluster_version_t::CLUSTER, made version_number_recognized_compatible use CLUSTER_VERSION_STRING, removed useless CLUSTER_VERSION constant.