add raft_leader_info command to always display output of current leader

output:

[root@quark-2 /]# redis-cli -c -p 7777
127.0.0.1:7777> raft_leader_info
-> Redirected to slot [0] located at quark-1.testnet:7777
 1) TERM 1
 2) LOG-START 0
 3) LOG-SIZE 28
 4) LEADER quark-1.testnet:7777
 5) CLUSTER-ID 3d659c1a-e70f-43f0-bed4-941a2ca0765b
 6) COMMIT-INDEX 27
 7) LAST-APPLIED 27
 8) BLOCKED-WRITES 0
 9) LAST-STATE-CHANGE 314 (5 minutes, 14 seconds)
10) ----------
11) MYSELF quark-1.testnet:7777
12) STATUS LEADER
13) ----------
14) MEMBERSHIP-EPOCH 0
15) NODES quark-1.testnet:7777,quark-2.testnet:7777,quark-3.testnet:7777
16) OBSERVERS
17) ----------
18) REPLICA quark-2.testnet:7777 ONLINE | UP-TO-DATE | NEXT-INDEX 28
19) REPLICA quark-3.testnet:7777 ONLINE | UP-TO-DATE | NEXT-INDEX 28