OpenOptions is high-level options clients give to ZODB when they want to
open a high-level IStorage, and not all of those options apply to drivers
- for example NoCache is meaningless since cache is provided by ZODB
infrastructure, not a driver.

On the other hand we are going to introduce driver-specific options,
that either low-lever users, or ZODB infrastructure itself will use when
opening drivers.

For this reasons decouple driver-specific options from OpenOptions into
DriverOptions.