plotextractor: configurable sleep timer

• Adds a new configuration parameter that allows control over the timeout period between file harvests of remote sites.

• The download() function (and consequently harvest_single()) will no longer enforce any timeout period, as it is only applicable in batch download use cases, leaving any flow-control resposibilities to the caller of these functions when used in such a way.
  
  Consequently, the plotextractor 'batch-jobs' are now enhanced with simple flow-control, alongside any harvesting tasks in BibHarvest using the plotextractor API. They all will use the above timeout parameter.

• Specific parameters meant to be configurable for the plotextractor module is now moved to invenio.conf, away from plotextractor_config.