Removed conservative option from store.update

This option didn't make much sense, and it was preventing from properly
synchronizing files when `update_stores --keep` was being used — it kept
the obsolete units around, without marking them as obsolete, thus extra
"garbage" strings (not present in the files anymore!) were being treated
as part of the DB stores.

Fixes bug 2963
Fix bug 2870

When making a search query, we first split the search string into
individual words, but for some reason if the resulting string contains
on the stopwords the searcher fails to find any results. Making a query
of the whole search string seems to fix then since PyLucene will
tokenize string itself and discard the stopwords. The resulting queries
still match substrings, reordered strings etc. so no regression is
expected.