This deprecates the Event constructor and the ``ts`` parameter of Enqueue()
Instead, versions are introduced that take a detail::MetadataVectorPtr which
can hold the network timestamp metadata and is meant to be allocated by the
caller instead of automatically during Enqueue() or within the Event
constructor.
This also introduces a BifConst ``EventMetadata::add_network_timestamp`` to
opt-in adding network timestamps to events globally. It's disabled by
default as there are not a lot of known use cases that need this.
Discussed with @J-Gras, calling Broker::publish() within a scheduled
should use the "intended timestamp" implicitly.
This is subtle, but supposedly more expected when running
a pcap replay cluster.
