A graceful shutdown

[unixpi]

Is your feature request related to a problem? Please describe.
When attempting to shutdown using Ctrl+c, Shutting down after having received SIGINT is hanging for long enough that I'm exiting the terminal window and opening up a new one to restart.

Describe the solution you'd like
A graceful shutdown.

Describe alternatives you've considered
None.

Additional context
BeaconNode.init() ignores the flag set by our Ctrl+c handler (it doesn't take control of the event loop). As long as this is the case, we can't do a graceful shutdown. See

nimbus-eth2/beacon_chain/beacon_node.nim

Line 91 in 7294d98

proc init*(T: type BeaconNode,

The bigger the database, the longer the shutdown takes. It can take a couple of hours on a recent Medalla db.

Stefan's proposed solution
Move most of the stuff from BeaconNode.init() to BeaconNode.run() and make it use that managed event loop. Some extra flags to order async steps are required.