generate load_id in extract step

[rudolfix]

Background
We generate load_id during normalization but we should do it in extract step. This will improve data lineage - all step infos will be correlated with it. It also allows to refactor step infos (like LoadInfo) to have the same structure created with load_id as primary identifier.

Currently the load_id is a string but we use unix timestamp of the time at which the package was created. We'll keep it that way but cleanup the code - and possibly let users override that.

Also note that currently the normalize step is forming packages by grouping them by schema name. If many extract steps were done for a single schema and then one normalize step: there will be a single load package. We want to preserve this behavior - even if this complicates our code a little.

Tasks

1. • in ExtractorStorage rename create_extract_id to create_load_id and use now() to create load_id.
2. • if we create load id for a schema with name for which load id is already present, reuse existing. this must happen via lookup into existing file structure (mechanism must survive restarts).
3. • we change normalize.py so it works with load packages created in extract step, currently we copy all files into a flat structure and ask normalize to group those by schema. now we have grouping already done so we just reuse it.
4. • methods in Pipeline that list extracted files must be revised and modeled after methods to list normalized and loaded packages. we may keep list_extracted_resources that produces a flat list of files for backward compatibilty.
5. • NormalizeStorage and TParsedNormalizeFileName should be refactored (ie. parse methiod) and we should bump version of normalize storage because we change the internal file layout in it