complib/timer: fix race accessing timer fields

complib/timer: fix race accessing timer fields

The timeout_time field of cl_timer_t is modified in the timer
callback function and also in calls to cl_timer_start, cl_timer_stop,
and cl_timer_trim.  The user cannot protect against the changes
made in the timer callback function, so the timer must provide
this protection itself.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>