Missing locks in the JobTimer class.

The JobTimer methods JobTimer.datetime() and JobTimer.timedelta() are currently not acquiring the lock. In rare cases of context switching this allow false values for the scheduled datetime of a JobTimer. Because of the Scheduler and Job architecture the correct execution of the planned jobs is not affected.

However, calling the functions Job.datetime() and Job.timedelta() might return incorrect values.

This issue can be fixed by acquiring the lock in the two functions.