Feat/membership notrack refactor (#17)

* refactor, minor fixes

* do not reset ongoing grace period on extension (cf. spec change)

* minor renaming, comments

* refactor, group functions by funcitonality

* extract membership expiration calculation to internal function

* save active duration per membership (must carry over after extension)

* optional lazy erasure from membership set

* minor fixes in tests

* fix: off-by-one: end index can't be equal to next free index

* minor refactoring, comments

* define period boundaries: start inclusive, end exclusive

* separate eraseMembership functions to user-focused (lazy) and admin-focused (tree cleanup)

* minor fix to maintain line lengths

* unify membership-related events

* add test for  zero grace period

* fix typo in comment

Co-authored-by: richΛrd <[email protected]>

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Co-authored-by: richΛrd <[email protected]>