use sanitizers instead of valgrind

Edwin & Michael suggest that using ASan (GCC & Clang) and MSan (Clang) is now better (catch more issues faster) than valgrind. I'm not that sure about it in the case of Spot since we do not run valgrind on all tests, but just on a large subset.

Adding -fsanitize=... to some build is not difficult, but we need a configure --disable-valgrind option so that we do not run valgrind and a sanitizer at the same time.