Introduce net.cluster

It's a module that provides connection to all replicasert of the
cluster. It maintains a list of connections (that actually are
net.replicaset connections) and allows to subscribe to an event
(aka watch) on entire cluster.

See module header for more detailed description.

src/box/CMakeLists.txt

@@ -35,6 +35,7 @@ if(ENABLE_INTEGRITY)
 endif()
 lua_source(lua_sources lua/net_box.lua net_box_lua)
 lua_source(lua_sources lua/net_replicaset.lua net_replicaset_lua)
+lua_source(lua_sources lua/net_cluster.lua net_cluster_lua)
 lua_source(lua_sources lua/upgrade.lua upgrade_lua)
 lua_source(lua_sources lua/console.lua console_lua)
 lua_source(lua_sources lua/xlog.lua xlog_lua)

src/box/lua/init.c

@@ -101,6 +101,7 @@ extern char session_lua[],
 #endif
 	net_box_lua[],
 	net_replicaset_lua[],
+	net_cluster_lua[],
 	mkversion_lua[],
 	upgrade_lua[],
 	console_lua[],
@@ -227,6 +228,7 @@ static const char *lua_sources[] = {
 	"box/upgrade", NULL, upgrade_lua,
 	"box/net_box", "net.box", net_box_lua,
 	"box/net_replicaset", "internal.net.replicaset", net_replicaset_lua,
+	"box/net_cluster", "experimental.net.cluster", net_cluster_lua,
 	"box/console", "console", console_lua,
 	"box/load_cfg", NULL, load_cfg_lua,
 	"box/key_def", "key_def", key_def_lua,