Turn compiler warnings into errors.

[robertnishihara]

Given problems that people are running into on nonstandard platforms such as #1008, we should make sure to get rid of all compiler warnings and cause them to fail the build. That will help catch bugs. We should also be strict about using static_cast and things like that.

[robertnishihara]

Some of this is done in #1116. We should probably also compile with -Wextra and -pedantic (see the discussion in #1116), but those add a lot of errors, and we'd need to exclude checking of the UT files and the redis code.