Rescheduling of all existing github pull-requests after restart of the master

[TeoYordanov]

We are facing an issue where all existing pull-requests in the project are getting rescheduled for build by the GitPoller after a restart of the master. This issue has always been present inside the 3.4.0-1build1 version of the Buildbot, but until recently we only had a project with a small amount of pull-requests being tracked by the poller. After adding our main project to be tracked by the GitPoller, this issue has caused over 3000 pull-requests (merged, closed, open) to be rescheduled after restarting the buildbot with its normal configuration (buildbot --restart path/to/the/master). We are using sqlite as our database system. I can provide more information if needed to resolve this case.