logImportWarning: temporarily disable logging, since we do not create log tables for the temp advimport tables

MapFields/processAllItems: only process items with import_status=0 (useful for when re-importing items that had errors, and we want to ignore items already successfully imported)

MapFields/processItem: silently ignore items re-processed (they may be in a batch with errors being fixed/re-imported)

Implement feature to allow re-importing values with errors (i.e. to replay the import with the errors).

This can be useful when debugging code for a custom helper/import,
or eventually when the error-viewer lets users fix values.

processItem: do not throw exceptions for items already processed (most often a browser page reload).

* Remove requirement to create a new API for importing, instead adding
  a processItem() method to the helper class.

fmrq/civicrm#45 Track import tasks in a dedicated tables (not Activities) and disable logging during the import to temporary table

fmrq/civicrm#44 MapFields: restructured so that the advimport tmp table becomes a bit more permanent so that we can track errors directly in it (and not duplicate data in the civicrm queue)