Return more information when opening an Environment

[Kerollmops]

When we open an environment the map size we specify is not the same as the one LMDB has internally. LMDB will silently clamp the size to the maximum between the one in the options and the one disk. We could get the Env::map_size and return it if required.

We currently disallow opening an environment that doesn't have the same EnvOpenOptions, and therefore map size. We should either:

• not store the map size in the EnvOpenOptions
• Return the EnvOpenOptions for the user to try again with them, and therefore add getters.
• Get the Env::map_size, store it in the global map and return it to the user when opening the env.