The safelog Write function can be called from multiple go routines, and
it was not thread safe. These locks in particular allow us to pass the
logscrubber's output io.Writer to other libraries, such as pion.
Our log scrubber was a bit over-zealous and was scrubbing timestamps as
well. While we're still over-scrubbing, we're now more precise and
ensure that compressed IPv6 addresses have "::"
