Introduce the Env::nested_read_txn from an RwTxn

[Kerollmops]

This PR is an attempt at patching LMDB to be capable of initiating multiple RoTxn from an RwTxn and, therefore, be able to read the content of an uncommitted RwTxn in parallel. You can find the LMDB patch in the dedicated fork. We can run many algorithms in Meilisearch to process the uncommitted changes and generate new data structures before committing the transaction.

This PR is open, but we also have it on crates.io under the v0.22.1-nested-rtxns-6 version. This version is experimental and will not be maintained in this state until we merge the changes into upstream LMDB. For the time being, we will continue to maintain it in this manner.

// opening a write transaction
let mut wtxn = env.write_txn()?;

// [use the RwTxn to write into the env]

// opening multiple read-only transactions without committing beforehand
let rtxns = (0..1000).map(|_| env.nested_read_txn(&wtxn)).collect::<heed::Result<Vec<_>>>()?;

// [use the RoTxns and move them onto different threads]