ltsmin: update usage of brick-hashset
From a discussion with Petr Rockai: "the possibly non-intuitive bit that you probably didn't notice is that the hashset is supposed to be passed to each thread by value. The copy semantics of the entire hashset are that of a shared pointer: multiple copies share the same underlying data. Each thread *must* have its own private copy of the hashset object (there are bits of state that each thread needs for bookkeeping and those must not be shared)." * spot/ltsmin/ltsmin.cc: here.