During network discovery idle discoverers do not pick up next available discovery rules.

Steps to reproduce:

• Start Zabbix server with StartDiscoverers=3 and Timeout=30
• Create discovery rule to scan /24 network with interval 1h any SNMP OID to check
• Add 3 more rules for other /24 networks with same interval and with only ICMP ping checks

Observed:
After 2 discoverers completed discovery rules #2 and #3 they will be idle and discoverer processes load will be 33%, but we still have unprocessed rule #4. This rule will be started by process 1 only after it will complete long term rule #1.

Expected:
Next available discoverer process should pick up next available discovery rule