Make the user cache case-insensitive (ASCII)
Make the user cache case-insensitive (ASCII)

Conduit is case-insensitive when doing a user lookup, but the cache
that maps userName to phid currently wasn't. This change makes
the cache case-insensitive assuming ASCII usernames (non-ASCII
usernames not supported as per https://secure.phabricator.com/T6003)

Test plan:

Conduit is case-insensitive:
    $ echo '{"usernames":["yli989"]}' | arc call-conduit user.query \
        |  jq  '.response[0].userName,.response[0].phid'
    "YLI989"
    "PHID-USER-2cx6yud27iadd6i6xl75"

arcyon was case-sensitive:
    $ arcyon query --authors yli989
    Traceback (most recent call last):
      File "/home19/phab/phabricator-tools/bin/arcyon-actual", line 34, in <module>
        sys.exit(aoncmd_arcyon.main())
      File "/home19/phab/phabricator-tools/py/aon/aoncmd_arcyon.py", line 107, in main
        return args.func(args)
      File "/home19/phab/phabricator-tools/py/aon/aoncmd_query.py", line 330, in process
        d = _process_user_fields(user, conduit, args)
      File "/home19/phab/phabricator-tools/py/aon/aoncmd_query.py", line 303, in _process_user_fields
        d[key] = [user_phids.get_phid(u) for u in d[key]]
      File "/home19/phab/phabricator-tools/py/phl/phlcon_user.py", line 74, in get_phid
        return self._user_to_phid[user]
    KeyError: 'yli989'

arcyon is now case-insensitive:
    $ arcyon query --authors yli989
    1015943 / Needs Review / review title

    $ arcyon query --authors YLI989
    1015943 / Needs Review / review title

aevri/phabricator-tools,aevri/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,bloomberg/phabricator-tools,bloomberg/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools