Avoid unnecessary db query on index update

pks_seen is only needed if objects are removed from
index, so only compute it if necessary.
Improve pks_seen to not build an intermediary list.
Merge branch 'master' into avoid-pks-seen-on-update

Change smart_text into smart_bytes as in master

Conflicts:
	haystack/management/commands/update_index.py
Merge pull request #775 from stefanw/avoid-pks-seen-on-update

Avoid unnecessary, potentially huge db query on index update