Added a check for the fields primary storage column as it's not always 'value'

The findBy method assumed a field 'value' for storage. This change gets the primary column for the field and checks that instead.
Observation, looking at link and term fields. EFQ isn't very good with OR so couldn't use all columns, unless we have an additional param that overrides the column choice. e.g. find_by_field_somefield('www.google.com', 'url');