core: whenever a unit terminates, log its consumed resources to the journal

core: whenever a unit terminates, log its consumed resources to the journal

This adds a new recognizable log message for each unit invocation that
contains structured information about consumed resources of the unit as
a whole after it terminated. This is particular useful for apps that
want to figure out what the resource consumption of a unit given a
specific invocation ID was.

The log message is only generated for units that have at least one
XyzAccounting= property turned on, and currently only covers IP traffic and CPU
time metrics.