Speed up Trevis CI

Avoid a couple of redundant rebuilds and compile
with 2 threads since travis gives 2vCPUs.

Also enable -O1 optimization for valgrind and
sanitizers, it should be safe withouth false
positives and it gives a very sensible speed
up, especially with valgrind.

The spee dup allow us to increase testing to
depth 10, useful for thread sanitizer.

No functional change.