Turn down logging in production builds

• We now make sure across all crates that any log lines about individual connections are sent only at trace level.
• The onionmasq-mobile crate now will only enable this level if you have the verbose feature flag set.
• This feature flag is not on by default, but can be during compilation (build-ndk.sh now supports an environment variable to set it).
• Together, this ensures that we don't avoid endangering users' privacy by spamming connection-level debug logging to the potentially widely readable system logcat.

Fixes #42 (closed).