The core package contains the CN-Infra Core that manages the startup
and graceful shutdown of CN-Infra based applications. The startup &
shutdown lifecycle is depicted in the sequence diagram below. The startup
and shutdown behavior is described in comments for the Start() and
Stop() functions in agent_core.go, and for the
EventLoopWithInterrupt()function in event_loop.go.
The core package also defines the CN-Infra Core's SPI
that must be implemented by each plugin (see Guidelines).
The SPI is used by the Core to Init(), AfterInit() and Close() each plugin.