Introduce better support to acquire global database handles

[Kerollmops]

Fixes the first point of #144 which is:

The database handle will be private to the current transaction until the transaction is successfully committed. If the transaction is aborted the handle will be closed automatically. After a successful commit the handle will reside in the shared environment, and may be used by other transactions.

About the second point below, we should only allow write transactions to create or open databases. It would be valid to open all the available databases at the first env opening, they are listed in the main database (name = NULL), and these names can be mixed with data. The issue is that heed is a typed key-value store, we must ask the user for the type of a database, maybe with a function given to the EnvOpenOptions.

[mdb_dbi_open] must not be called from multiple concurrent transactions in the same process. A transaction that uses this function must finish (either commit or abort) before any other transaction in the process may use this function.