Guide: Impose limits on the number of messages that a candidate can send

[pepyakin]

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[pepyakin]

In the current incarnation of the Router module it is a bit vague what exactly happens if either config.watermark_upward_queue_size or config.max_upward_queue_count are exceeded.

polkadot/roadmap/implementers-guide/src/runtime/router.md

Line 35 in 482abf8

1. Dispatch queued upward messages from `RelayDispatchQueues` in a FIFO order applying the `config.watermark_upward_queue_size` and `config.max_upward_queue_count` limits.

My current thinking is the candidate acceptance function should fail if any of those limits are exceeded. Additionally, in light of #1539 these conditions should be checkable using only NonPersistedValidationData.