Event mechanism to initiate update runs

[benmaddison]

Currently, to start a new update run before the refresh-interval timer fires,
the daemon has to be killed with agent PrefixListAgent terminate.

This is messy and unnecessary.

Until a pub/sub mechanism is available via SysDB/EOS-SDK, create a socket on
agent start up that can be used (e.g. by CLI handlers) to send events/RPCs
directly to the agent.

The initial usecase will be to implement a clear prefix-list-agent CLI
command that will clear the current timer and initiate an update run
immediately.