fix: live-array delete sync should not clear the set on length match

[runspired]

test from #8926
resolves #8924

adds test for unloadAll not fully cleaning up

The rough issue is that if all records are removed and the same number re-added before the array has synced we were then fully clearing membership which would make the next sync after this fail to work properly.

e.g.

arr.length; // 2
arrSet.size; // 2
<add> C
<add> D
<rem> A
<rem> B

if (rem.length === arr.length) { arrSet.clear() }

arr.length; // 2
arrSet.size; // 0