Zabbix Agent 2 send Heartbeat-messages every 15 seconds regardless of HeartbeatFrequency value if host in ServerActive is unavailable.

This is a problem in my case because it leads to 4 times increase of A/AAAA-requests to DNS with HeartbeatFrequency being set to 60 (by default).

Steps to reproduce:

1. Change ServerActive value in config to nonexistent host (no matter IP or hostname).
2. Change DebugLevel value in config to 4 (debug).
3. Restart Zabbix Agent 2.
4. Grep (-i) your agent log for messages containing sendHeartbeatMsg substring.
5. Check interval between consequent End of sendHeartBeatMsg() message (N-1 attempt) and In sendHeartbeatMsg() message (N attempt).

Result:
Interval between consequent End of sendHeartBeatMsg() message (N-1 attempt) and In sendHeartbeatMsg() message (N attempt) equals to ~15 seconds.

Expected:
Interval between consequent End of sendHeartBeatMsg() message (N-1 attempt) and In sendHeartbeatMsg() message (N attempt) equals to value specified in HeartbeatFrequency.