Add parallel LMDB database implementation

[casey]

Refactor redb into a separate module, so we can add a parallel lmdb implementation. I don't think there's a strong reason to merge this as is, it's more of a starting point to add an lmdb database implementation. We don't need a trait, we just need to add an lmdb_database.rs source file containing Database and WriteTransaction with the same public functions.

[casey]

@terror I refactored things so it should be straightforward to add an lmdb database implementation, just turn on the lmdb feature and start implementing the todos in lmdb_database.rs

[casey]

I didn't see anything that jumped out at me as being clearly wrong. Do things like cargo run info work?

[casey]

@terror I figured it out! Lmdb (like redb) only allows one writer at a time. Calling Environment::create_db must count as a write, so if there's already a write transaction, Environment::create_db will block. The solution here is to create the databases before opening the write transaction.

[casey]

I added some more changes which make the empty_index test pass. Putting open_db after begin_ro_txn failed, and the reverse worked, which surprised me, since I assumed the problem was with doing this with write transactions.