When closing an `Env` we must close the databases

[Kerollmops]

https://github.com/meilisearch/heed/blob/dea2f8506692f636dd11bfa0473434c9a040f7e5/heed/src/env.rs#L319-L334

[Kerollmops]

It doesn't seem required to close the DBIs when you want to close an environment:

• The mdb_env_close0 (called by mdb_env_close) is already freeing the DBIs for us.
  • The mdb_dbi_close works the same as the mdb_env_close0 function.