Rust-rocksdb: TxnDB : Transactions via rocksdb_transaction_t

Transaction functionality is exposed in rocksdb via a parallel implementation of the rocksdb_t called rocksdb_transaction_t . Now that we have bindgen (#143) we get access to it for "free".

First, for brevity, I propose a new acronym that is the RocksDB parallel of CRUD, I call it
PIGMED , that's Put Iterate MErge Delete. Not catchy, I know.

The question is how to best expose the Transaction functionality while :

1. Not duplicating the entire DB impl for TxnDB
2. Maintaining type safety.

There are quite a few new methods for TxnDb that would need to be wrapped, some of them might make sense as guards or even closures.

I don't think there is a way to unify the PIGMED operations of DB and TxnDB while retaining type safety or even ergonomics. But there isn't actually a ton of PIGMED functionality in DB.

I propose we move all of the utility/environment functionality, that is anything that doesn't require a rocksdb_t into a separate utility module or modules (e.g. rocksdb::cf::create for rocksdb_create_cf)

We then slim down DB to just the PIGMED operations.

My hope is that we would be able to leverage the same Iterator functionality. There is a rocksdb_transaction_create_iterator but the iterator_t is the same.

Thoughts?