feature: Implement db:"field_name,noindex,omitmepty" tags

[trakhimenok]

Currently to mark a field as non indexed or omitempty it's required to use platform specific tags like:

type SomeEntity struct {
  SomeField string `datastore:"some_field,noindex,omitempty"`
}

Ideally we should have strongo/db tags that would be translated to desired required by specific adapters (like gaedb).

type SomeEntity struct {
  SomeField string `db:"some_field,noindex,omitempty"`
}

Probably DB adapters should be required to implement some interface to make sure the expected behavior is not being ignored.