Converts ingest staging in external content manager to the new tag-based staging API

moved tag and irods protocol handlers to staging-areas project
added staging-areas support to IrodsExternalContentManager for reading tag, irods, etc..
added unit test for fcrepo reading tag URIs

implemented external content fetching via the staging api and config
let tag uris through the mets validation process

fixed tag file retrieval
added tag: protocol to METS schematron

fixed warning imports