Port server's log sanitization to client, broker, and proxy-go

legacy/trac#21304 (moved) added a log sanitizer to the server (bridge) code that searches for IP addresses in logs and elides them. We noted in comment:17:ticket:21304 that the other components--client, broker, and proxy-go--can benefit from the same log sanitization.

comment:18:ticket:21304 suggests a way to do it: move the logScrubber code into a new top-level subdirectory and safelog package, and have the other programs import git.torproject.org/pluggable-transports/snowflake.git/safelog.