raft: introduce box.info.raft

Box.info.raft returns a table of form:

    {
        state: <string>,
        term: <number>,
        vote: <instance ID>,
        leader: <instance ID>
    }

The fields correspond to the same named Raft concepts one to one.
This info dump is supposed to help with the tests, first of all.
And with investigation of problems in a real cluster.

Part of #1146

src/box/lua/info.c

@@ -49,6 +49,7 @@
 #include "main.h"
 #include "version.h"
 #include "box/box.h"
+#include "box/raft.h"
 #include "lua/utils.h"
 #include "fiber.h"
 #include "tt_static.h"
@@ -577,6 +578,21 @@ lbox_info_listen(struct lua_State *L)
 	return 1;
 }
 
+static int
+lbox_info_election(struct lua_State *L)
+{
+	lua_createtable(L, 0, 4);
+	lua_pushstring(L, raft_state_strs[raft.state]);
+	lua_setfield(L, -2, "state");
+	luaL_pushuint64(L, raft.volatile_term);
+	lua_setfield(L, -2, "term");
+	lua_pushinteger(L, raft.volatile_vote);
+	lua_setfield(L, -2, "vote");
+	lua_pushinteger(L, raft.leader);
+	lua_setfield(L, -2, "leader");
+	return 1;
+}
+
 static const struct luaL_Reg lbox_info_dynamic_meta[] = {
 	{"id", lbox_info_id},
 	{"uuid", lbox_info_uuid},
@@ -595,6 +611,7 @@ static const struct luaL_Reg lbox_info_dynamic_meta[] = {
 	{"vinyl", lbox_info_vinyl},
 	{"sql", lbox_info_sql},
 	{"listen", lbox_info_listen},
+	{"election", lbox_info_election},
 	{NULL, NULL}
 };
 

test/box/info.result

@@ -75,6 +75,7 @@ table.sort(t)
 t
 ---
 - - cluster
+  - election
   - gc
   - id
   - listen