[#6541] Added import_id for GC tracker importer

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6541] Refactored import_id and ImportIdConverter

Made default import_id implementation use a dict to
record source and other info for use with multiple importers.

Requires Ming 0.3.9 (from 0.3.x branch) for MIM bug fixes for
tests to pass.

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>