PER-25: prevented msg_resource to be populated during a regular request

This is to prevent checking for messages of RL modules: user.options and user.tokens, which is completely unnecessary. Moreover it happens on every page render, which kills database when msg_resource gets truncated by any mean.

This is a little bit hacky solution, however I tried to optimize it for future core upgrades and to make them easier and more straight-forward. If you see any improvement please leave a comment.

If no objections are places here till tomorrow I'm going to merge it to release branch, push a production and test it by truncating msg_resource on production (it doesn't cause any data loss) and see how it helps in such case.

@owend @Wyvek @macbre I'd like you to take a look at this changeset.