Track users by name instead of UIDs

For non-munge use, display the user supplied username. For munge usage,
convert uid to string.

Makes for much more informative log messages especially when not using
munge.

Unfortunately, despite avoiding copying, using strings as user-id seems
to cause memory corruption after about five minutes of intense usage.
Investigation pending.

Change-Id: If4626e078a41ce8d4b7059492fea1137db94bfa4