Enable -fsanitize flags in Travis

[eklitzke]

This idea is from discussion in #12686 . GCC 4.8 supports two -fsanitize flags, -fsanitize=address and -fsanitize=thread (and newer versions support even more of these flags). It would be a good idea to enable these options in Travis. @sipa points out that the build still isn't completely clean when using these flags, so to add this we'd either have to fix all of the existing errors or some up with a whilelisting mechanism to grandfather the old violations.

[eklitzke]

I'm trying to add a configure option for this in #12692 .

I tested the Bitcoin test suite with various sanitizer configurations, and the Bitcoin test suite passes cleanly with the thread and undefined sanitizers, but there are serious issues with the address sanitizer that need to be resolved. The address sanitizer injects a segfault when errors occur, so enabling this in Travis is an all-or-nothing kind of thing.

[maflcko]

Since some of the sanitizers are exclusive, we could enable one of the sanitisers for each of the travis jobs that run the functional test suite?

[maflcko]

The thread sanitizer fails again after #13247. It could make sense to fix that issue and then set --with-sanitizers=thread for the x86_64 Linux, No wallet travis job, which runs the unit tests, but not the functional tests.

Same for the undefined sanitizer.

[Empact]

Addressed in #14794 and #14829

Could close?