Deprecate dbDriver

[hadley]

So we can standardise on one way of connecting to a database: dbConnect(RSQLite::SQLite())

[krlmlr]

@hadley: I wonder if there's a good way to infer the constructor just from the package name or environment. Maybe provide a registration entry point in DBI?

[hadley]

I don't think adding another way to get the database driver is a good solution here. IMO it's better to be explicit.