zodb/storage: Delegate zurl opts extract to drivers

We can't extract storage client options in general-purpose zodburi
opener, because zodburi aren't standardized. Each storage may define its
own syntax, therefore it is better if this is done in each local storage
driver.

go/zodb/storage.go

@@ -106,10 +106,6 @@ func OpenDriver(ctx context.Context, zurl string, opt *DriverOptions) (_ IStorag
 		return nil, InvalidTid, err
 	}
 
-	// XXX commonly handle some options from url -> opt?
-	// (e.g. ?readonly=1 -> opt.ReadOnly=true + remove ?readonly=1 from URL)
-	// ----//---- nocache
-
 	opener, ok := driverRegistry[u.Scheme]
 	if !ok {
 		return nil, InvalidTid, fmt.Errorf("zodb: URL scheme \"%s:\" not supported", u.Scheme)