approval-distribution: limit the amount of assignments on unify (#2737)

* approval-distribution: limit the amount of packets on unify

* guide: fix a typo

* compilation fix

* grammar

* Update roadmap/implementers-guide/src/node/approval/approval-distribution.md

Co-authored-by: David <dvdplm@gmail.com>

* more grammar

* propagate only local assignments/approvals after a certain depth

* increase the threshold

* guides update

Co-authored-by: David <dvdplm@gmail.com>
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