Fix alerts duplication in HA cluster mode

[nip-was-here]

#4152

There is fix for duplicates in HA cluster mode & reduce for broadcasting several times exactly same state.

!cluster.OversizedMessage(b) is not necessary here:

On current release version (0.27.0)

• Using broadcast
• Setup broadcast func
• Call of SetBroadcast
• Setup Channel Broadcast
• Inside of AddState
• Broadcast func for Channel
• Choose way for send data in func
• Small state broadcasting
  • In Broadcast func
  • Setup send func for Channel with memberlist lib queue
  • Used func in memberlist lib
  • Preparing data func in queue
  • Using prev func
  • Using prev func
  • Using func to send by UDP
  • Direct mention of UDP
• Big state broadcasting
  • In Broadcast func
  • Setup send func for Channel with memberlist func SendReliable
  • Used func in memberlist lib
  • Using func to send by TCP
  • If going deeper

To sum up: at step Choose way for send data in func there's same check with choosing way of sending data - TCP or UDP

For testing this fix, stand was created:

• Several vmalert instances send alerts to 2 HA clusters of Alertmanager - with patch & without it
• Each Alertmanager cluster know only about his own nodes
• Both clusters of Alertmanager send to one self-developed receiver
• Receiver calculates duplicates of alerts with delta < 1 minute and other stats

Results of tests with big by body incoming alerts:

{
  "main": {
    "alerts_last_minute": 2,
    "alerts_total": 10864,
    "duplicates_total": 2887
  },
  "patched": {
    "alerts_last_minute": 2,
    "alerts_total": 7579,
    "duplicates_total": 0
  }
}

[MarcWort]

I stumbled across this fix because we were sometimes duplicating notifications in Slack. Are there any plans to review this change?