timeval: Add Clang thread-safety annotations, fix unimportant violation.

Signed-off-by: Ben Pfaff <blp@nicira.com>
timeval: Restore ability to warp time forward when time is not stopped.

Commit 31ef9f5178dee18 (timeval: Remove CACHE_TIME scheme.) inadvertently
removed the ability to warp time forward, for use in tests, except when
time is stopped.  This fixes the problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>
timeval: Replace rwlock by mutex.

It's only held briefly now and in general a mutex tends to be preferred for
that case.

Signed-off-by: Ben Pfaff <blp@nicira.com>