pcap/file: fix race during pcap processing start

pcap/file: fix race during pcap processing start

A race condition during the start of pcap file processing could cause
missed alerts and logged events. This race happens between the packet
threads and the flow manager. It was observed on slower hardware, but in
theory could happen on any machine. It required the 'autofp' runmode.

In commit 6f560144c1b9 ("time: improve offline time handling") the logic
was added to make the flow manager use a minimum of all the packet threads
perception of time.

The race condition was that the flow manager may become active _before_
all of the packet threads have started processing packets and thus setting
their timestamp. The threads that had not yet initialized their timestamp
would not be considered when calculating the minimum.

As a result of this, older packets timestamps would not yet be registered.
This would give the Flow Manager a timestamp too far in the future. While
the FM was running, the packet processing would start and a flow would
be created. This flow would then immediately be considered 'timed out' by
the FM, due to the timestamp too far in the future.

In the observed case, the thread processing packet 1 from the pcap had not
yet started processing while other threads had already started. The FM was
also already active. Due to the timestamps in the pcap this meant that the
time the FM used was about 500 seconds in the future compared to packet 1.

This patch fixes the issue by initializing all of the threads timestamps
with the timestamp value of the first packet. This way the minimum will
always consider this timestamp.