Allow deferring fetching of metadata to cron

Problem/Motivation

When saving an entity with many file_links there might be a problem to fetch all the metadata before php times out.

Steps to reproduce

Create/update a node with file links and add a lot of links to documents hosted on a slow server.

Proposed resolution

Add a configuration setting to allow fetching the data as part of a cron job.

Remaining tasks

implement, test, etc

User interface changes

link data may not be updated immediately but wait for cron.

API changes

none

Data model changes

none