Support old-style item-level call-number fields

Although FOLIO's representation of item-level call numbers changed in November 2019, there are evidently still services out there returning the old-style information — and people are running the Z39.50 server against them now.

For the sake of such old services, the Z39.50 server should fall back to reading the old-style fields when the new ones are not around.

For background information, see Slack conversation with Sobha Duvvuri at
https://folio-project.slack.com/archives/DCBQHEK8S/p1621855008000300