Shutdown like a Sir (#1545)

## Issue Addressed
#1494 

## Proposed Changes
- Give the TaskExecutor the sender side of a channel that a task can clone to request shutting down
- The receiver side of this channel is in environment and now we block until ctrl+c or an internal shutdown signal is received
- The swarm now informs when it has reached 0 listeners
- The network receives this message and requests the shutdown

scripts/local_testnet/second_beacon_node.sh

@@ -16,5 +16,6 @@ exec lighthouse \
 	--testnet-dir $TESTNET_DIR \
 	--dummy-eth1 \
 	--http \
+    --port 9902 \
 	--http-port 6052 \
 	--boot-nodes $(cat $BEACON_DIR/beacon/network/enr.dat)