Added multithreading to fetcher.

NYPL is currently the only profile for which it's enabled.

Also includes changes to:
- couch:  added fetcher_threads to ingestion document
- AbsoluteUrlFetcher.fetch_all_data(): added "set" param.
  - for page numbering in NYPL, IA
- amara.thirdparty.httplib2 has been superceded by
  threadsafe standard library httplib in fetcher
And:
- Restored field removed from DONT_STRIP_DOT_END in commit 66fd47fa3
- Changed NYPL test: set travis_exclude flag and do not fetch the world