traffic_replay: Improve user generation debug

traffic_replay: Improve user generation debug

When creating 1000s of users you currently get a lot of debug, but at
the same time you have no idea how far through creating the users you
actually are.

Instead of logging every single user account that's created, log every
50th (as well as how far through the overall generation we are).

Logger already includes timestamps, so we can remove generating the
timestamp diff manually. User creation is the slowest operation - adding
groups/memberships is much faster, so we don't need to log as
frequently.

Note that there is a usability trade-off on how frequently we log
depending on whether the user is using the slower (but more common)
method of going via LDAP, vs the much faster (but more obscure) method
of writing directly to sam.ldb with ldb:nosync=true. In my tests, we end
up logging every ~30-ish secs with LDAP, and every ~3 seconds with
direct file writes.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>