Fix rare duplicate time issue On systems with imprecise clocks

Fix rare duplicate time issue On systems with imprecise clocks
(e.g. windows), there was as issue where microsecond rollover could
conceivably cause the same time to be reported twice. Also document
potential performance improvement by using thread-local storage for
last_time and eliminating the mutex.

Signed-off-by: Kevin Wasserman <kevin.wasserman@painless-security.com>
Signed-off-by: Sam Hartman <hartmans@painless-security.com>
git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25088 dc483132-0cff-0310-8789-dd5450dbe970