Rework the fts5 db-close cleanup to avoid that fts5 internally calls a just-unbound...

Rework the fts5 db-close cleanup to avoid that fts5 internally calls a just-unbound-from-wasm xDestroy() method. There is still work to do here to cover the case of fts5 functions being replaced (in which all xDestroy methods have to be retained until the db is closed).

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