lib: thread-safe s-expression query parser

lib: thread-safe s-expression query parser

Follow-up of 6273966d, now that sfsexp 1.4.1 doesn't rely on globals
anymore by default (https://github.com/mjsottile/sfsexp/issues/21).

This simply defers the initial query generation to use the thread-safe
helper (xapian_query_match_all) instead of Xapian::Query::MatchAll.