Setting the trace clock source doesn't work

[ajbeamon]

fdbserver has an option to set the clock used for trace events to either use now() or a call to timer(). By default, it had been using the now clock.

A recent change of mine (#1875) made it so that the now clock was only used on the network thread to avoid unsafe accesses of global data from different threads. This change also had the unintended side effect of causing the clock source on the network thread to always revert to now, even if it had changed to realtime.

There was uncovered by a recent PR (#2329) to add the ability to change the clock source in clients. Changes here should consider the implications for that PR as well.

[ajbeamon]

Resolved by #2420