Change the RoTxn to support both WithTls and WithoutTls

[Kerollmops]

Currently, all RoTxn created structs are Send if the read-txn-no-tls crate feature is set. This is not ideal because TLS-based RoTxn are less expensive and could be enabled by a dependency also depending on heed.

The best way to disable TLS would be to create two new unit structs: the WithTls and WithoutTls. The RoTxn would be parametric on it and become: RoTxn<WithoutTls> which makes it Send. We define this parameter when constructing it.

Unfortunately, the Env::read_txn method cannot use the default WithoutTls parameter as there are no default generics on methods in Rust, yet...