Turn on warnings by default

[ArthurBandaryk]

We need to turn all warnings into errors (-Werror compiler option). It would make us fix all warnings except deprecated-declarations warning. For now we have warning about syscall, in future we are going to fix this too. You can see this issue on the screen below.

Thus, with the help of the command below we will be able to clean our builds from warnings (again: except syscall warning):
bazel build --copt="-Werror" --copt="-Wno-error=deprecated-declarations" :eventuals