Remove the special case of initializing network time if it hasn't
happened yet. The argument about broker.log containing 0.0 timestamps
is more a problem of the log, not something that would justify modifying
network time globally. For broker.log and possibly cluster.log, it might
be more reasonable to use current time, anyway.
I was a bit wary about tables backed by broker stores being populated
with network_time set to 0.0, but there seems to exist logic and assumptions
that this is okay: It should be the same as if one populates a table with
expirations set within zeek_init().
In fact, staring a bit more, *not setting* network time might be more correct
as workers that don't see packets would never set zeek_start_network_time
which is used within the expiration computation.
