Zabbix

........S. [ZBX-2806] reduce the amount of work on dbsyncer and fix
superseded escalations removal

dbsyncer process only adds 1 record to escalations table on every
event:

- escalation with eventid (start escalation)
- escalation with r_eventid (stop escalation, recovery)

escalator processes all escalations in every iteration, even the ones
with nextcheck in the future. This improves deletion of superseded
escalations and also allows to get last eventid when processing
recovery.

This commit also fixes db patches for escalations, during upgrade
escalations table is emptied.