Allow copying of lmdb::dbi instances about lmdbxx

[landtuna]

I think this issue makes the library hard to use in a multithreaded environment. The documentation for lmdb says of mdb_dbi_open, "After a successful commit the handle will reside in the shared environment, and may be used by other transactions." It also says, "This function must not be called from multiple concurrent transactions in the same process."

Together, I take it to mean that the main process should open a database once (inside a transaction) and then pass the dbi handle to each of the threads it spawns. Without being able to copy an lmdb::dbi, I think the only way to do this is to pass the underlying handle around.

[hoytech]

In my fork of this library (https://github.com/hoytech/lmdbxx) I have made lmdb::dbi instances copyable. I can't think of any situation where copying the dbi would cause a bug, and in many cases being able to copy dbis or pass them by value simplifies application code.