Prevent invalid up/downvotes when concurrent requests

Do not update up/downvotes counters server-side when up/downvoters have
been modified in the meantime. Ex: concurrent upvote and cancelUpvote
HTTP requests at once, when the user clicks button several times in a
row.
Merge pull request #416 from spifd/fix-concurrent-updownvotes

Fixes #415: prevent invalid up/downvotes when concurrent requests