Database picker fields use the wrong RestFieldOperationsHandler

Database picker fields don't specify their own handler for REST operations, which means that Jira applies the default which is TextCustomFieldOperationsHandler. This works out fine for single-picker fields, but means that multi-picker fields can't be supplied with multiple values via the REST API.

We already have a MultiIssuePickerOperationsHandler, so the precedent is there for us to add something similar for DB pickers.