API to manage database lifecycle and use handle outside of withDB bracket

[epoberezkin]

Currently there are two problems with the API:

• you cannot open database once, when the server starts, and close on exit, instead it would need to be opened and closed on every operation - this is likely to be inefficient.
• API doesn't differentiate between opening and creating database - this is error prone and it is better to manage in the application, where the application can potentially warn user about the missing database instead of creating a new one.

While the current withDB API needs to be preserved both for backwards compatibility and for simple testing, what is missing for production use is open/close/create APIs.

Happy to contribute if it's acceptable.

[epoberezkin]

https://github.com/simplex-chat/rocksdb-haskell/pull/1/files?diff=split&w=1 - this does it for withDB, a similar thing can be done to withDBCF, but more stuff will need to be held in database handle probably.

Let me know if you want this PR