Parametric TLS for read transactions

[Kerollmops]

This PR introduces a breaking change to the RoTxn type, allowing it to be either Send or !Send without toggling a feature flag. This is useful when using both sendable and non-sendable RoTxns in the same application. Non-sendable RoTxns are known to be faster as they require less to no allocation.

To Do

• Make it work with the EncryptedEnv.
• Document the WithTls and WithoutTls type parameters.
• Add tests to check that WithoutTls makes RoTxn Send and WithTls makes them !Send.
• Explain that it is tied to an Env and that it follows the following rules.
• (optional) Make it possible to recycle the RoTxn by using the mdb_txn_reset function.