This commit adds a sanitizer run script, and updates travis to
run it in our nightly builds. Valgrind has been very slow and
unstable, so much so that we only run it during pre-release
tests, and even then it doesn't necissarily provide useful
output. The sanitizer tests run about 10x faster, and seem
stable.
We currently only run ASan and UBSan, as MSan has too many
issues within postgres's own initialization functions. As MSan
cannot be added to a binary that enables ASan, it would require
separate jobs anyway.
