Avoid mutex locking in krb5int_trace()

Avoid mutex locking in krb5int_trace()

Trace logging doesn't need unique timestamps, so the locking within
krb5_crypto_us_timeofday() makes trace logging slower for no reason.
Add a new helper k5_us_timeofday(), which is merely a wrapper around
the existing get_time_now(), and use it in krb5int_trace().

[ghudson@mit.edu: edited commit message]