sentry: rateLimit type "uid"

[Ivshti]

allow rate limiting by session uid

this will be very similar to ip, except it wouldn't allow non-authenticated users to submit at all

this is not spec'd up, but it's very easy: the same as type ip but using sid ({ type: "sid", timeframe: .. })

should be implemented here: https://github.com/AdExNetwork/adex-validator-stack-js/blob/df625fecad988319bb46f40244820c6dc45586e0/services/sentry/lib/access.js#L45 using pretty much the same code