This change addresses a race condition where setnetworkactive=false wouldn't always disconnect all peers.
Before this change, the following could happen:
1. Thread A -- Begins connecting to a node.
2. Thread B -- Sets kNetworkActive=false and disconnects connected nodes.
3. Thread A -- Finishes connecting and adds node to list of connected nodes.
The node that was connected from Thread A remains connected and active,
even though kNetworkActive=false.
To fix the race, disconnections when kNetworkActive=false are now handled in the main network loop.
fixes#13038
lmanners