Handle gap events in batches, in case there are thousands

ee/changelogs/unreleased/tc-geo-gap-tracking-improvements.yml

+---
+title: Geo log cursor gap tracking performance improvements
+merge_request: 6754
+author:
+type: performance

ee/lib/gitlab/geo/event_gap_tracking.rb

@@ -52,10 +52,10 @@ module Gitlab
 
           gap_ids = redis.zrangebyscore(GEO_EVENT_LOG_GAPS, '-inf', grace_timestamp).map(&:to_i)
 
-          event_logs = ::Geo::EventLog.includes_events.where(id: gap_ids)
-          event_logs.each { |event_log| yield event_log }
-
-          redis.zrem(GEO_EVENT_LOG_GAPS, event_logs.map(&:id)) if event_logs.any?
+          ::Geo::EventLog.where(id: gap_ids).each_batch do |batch|
+            batch.includes_events.each { |event_log| yield event_log }
+            redis.zrem(GEO_EVENT_LOG_GAPS, batch.map(&:id))
+          end
         end
       end