Batched notifications job for the mentions is executed not frequently enough

Issue Summary

The recent changes in the batched notifications contain bug that prevents the emails containing mentions to be sent immediately.
This may also create a performance problems on bigger instances.

Steps to Reproduce

1. Set longer interval, e.g. 10 minutes on /secure/admin/ConfigureBatching!default.jspa
2. @mention somebody

Expected Results

The batched email containing mentions should be sent almost immediately (approx 1 minute).

Actual Results

The mention mail is sent after at most the configured interval period (at most 10 minutes).

Debug mail logging may be enabled to trace the batching mechanism in atlassian-jira-outgoing-mail.log

Workaround

Set smaller interval.