Ensure mutual exclusion on cluster nodes/slots update when starting benchmark

[filipecosta90]

Fixes issues like the one bellow:

$ ./pubsub-sub-bench -port 30001 --mode subscribe --subscribers-per-channel 9 --channel-minimum 1 --channel-maximum 100 --subscriber-prefix "channel-" --test-time 80 --oss-cluster-api-distribute-subscribers --json-out-file=pubresult.json
2023/06/15 14:47:40 pubsub-sub-bench (git_sha1:e0d8fdda3eeef18d5ee7c351082789e3169ff5b4-dirty)
2023/06/15 14:47:40 Reloading cluster state.
2023/06/15 14:47:40 Getting cluster slots info.
2023/06/15 14:47:40 Detailing cluster slots info. Total slot groups: 5
2023/06/15 14:47:40     Slot range start 0 end 3276. Nodes: [{cf91f803d988d4682a6f7e5595556fbe1a6c70d8 127.0.0.1:30001 map[]}]
2023/06/15 14:47:40     Slot range start 3277 end 6553. Nodes: [{6d372f3960660070100fd317f880602e620011f7 127.0.0.1:30002 map[]}]
2023/06/15 14:47:40     Slot range start 6554 end 9829. Nodes: [{7ad695ce75fbebe3d95876c1fb0b8034795f34e1 127.0.0.1:30003 map[]}]
2023/06/15 14:47:40     Slot range start 9830 end 13106. Nodes: [{577aaf1bca1fe84fa77f6f3f849850a704fc8c92 127.0.0.1:30004 map[]}]
2023/06/15 14:47:40     Slot range start 13107 end 16383. Nodes: [{ab1bf2bb2c904d07a7eb119d4d3b70cf0d20deb7 127.0.0.1:30005 map[]}]
2023/06/15 14:47:40 Detailing cluster node info
2023/06/15 14:47:40 Cluster node pos #2. Address: Redis<127.0.0.1:30005 db
2023/06/15 14:47:40 Cluster node pos #3. Address: Redis<127.0.0.1:30003 db
2023/06/15 14:47:40 Cluster node pos #3. Address: Redis<127.0.0.1:30001 db
2023/06/15 14:47:40 Cluster node pos #4. Address: Redis<127.0.0.1:30002 db
2023/06/15 14:47:40 Cluster node pos #5. Address: Redis<127.0.0.1:30004 db
2023/06/15 14:47:40 Total subcriptions: 900. Subscriptions per node 180. Total messages: 0
2023/06/15 14:47:40 Will use a subscriber prefix of: channel-<channel id>
2023/06/15 14:47:40 Setting per Node pool size of 180 given you haven't specified a value and we have 180 Subscriptions per node. You can control this option via --pool_size=<value>
2023/06/15 14:47:40 Reloading cluster state given we've changed pool size.
2023/06/15 14:47:40 Getting cluster slots info.
2023/06/15 14:47:40 Detailing cluster slots info. Total slot groups: 5
2023/06/15 14:47:40     Slot range start 0 end 3276. Nodes: [{cf91f803d988d4682a6f7e5595556fbe1a6c70d8 127.0.0.1:30001 map[]}]
2023/06/15 14:47:40     Slot range start 3277 end 6553. Nodes: [{6d372f3960660070100fd317f880602e620011f7 127.0.0.1:30002 map[]}]
2023/06/15 14:47:40     Slot range start 6554 end 9829. Nodes: [{7ad695ce75fbebe3d95876c1fb0b8034795f34e1 127.0.0.1:30003 map[]}]
2023/06/15 14:47:40     Slot range start 9830 end 13106. Nodes: [{577aaf1bca1fe84fa77f6f3f849850a704fc8c92 127.0.0.1:30004 map[]}]
2023/06/15 14:47:40     Slot range start 13107 end 16383. Nodes: [{ab1bf2bb2c904d07a7eb119d4d3b70cf0d20deb7 127.0.0.1:30005 map[]}]
2023/06/15 14:47:40 Detailing cluster node info
2023/06/15 14:47:40 Cluster node pos #2. Address: Redis<127.0.0.1:30005 db
2023/06/15 14:47:40 Cluster node pos #3. Address: Redis<127.0.0.1:30001 db
2023/06/15 14:47:40 Cluster node pos #4. Address: Redis<127.0.0.1:30004 db
2023/06/15 14:47:40 Cluster node pos #4. Address: Redis<127.0.0.1:30003 db
2023/06/15 14:47:40 Cluster node pos #2. Address: Redis<127.0.0.1:30002 db
panic: runtime error: index out of range [4] with length 4

goroutine 1 [running]:
main.main()
        /home/fco/redislabs/pubsub-sub-bench/subscriber.go:252 +0x2570