Fix discarding explicit routes while removing duplicate ones (#4414)

In the new clustering logic for v2.10, sometimes the `TestStressChainedSolicitWorks` 
test would flake because a node would end up with only implicit routes. In this change, 
we stamp that one of the remotes is configured so that the nodes at least have one explicit
configured remote node.

server/route.go

@@ -2089,8 +2089,9 @@ func handleDuplicateRoute(remote, c *client, setNoReconnect bool) {
 	// removeRoute() now does the right thing of doing that only when
 	// the closed connection was an added route connection.
 	c.mu.Lock()
-	didSolict := c.route.didSolicit
+	didSolicit := c.route.didSolicit
 	url := c.route.url
+	rtype := c.route.routeType
 	if setNoReconnect {
 		c.flags.set(noReconnect)
 	}
@@ -2101,10 +2102,15 @@ func handleDuplicateRoute(remote, c *client, setNoReconnect bool) {
 	}
 
 	remote.mu.Lock()
-	if didSolict && !remote.route.didSolicit {
+	if didSolicit && !remote.route.didSolicit {
 		remote.route.didSolicit = true
 		remote.route.url = url
 	}
+	// The extra route might be an configured explicit route
+	// so keep the state that the remote was configured.
+	if rtype == Explicit {
+		remote.route.routeType = rtype
+	}
 	// This is to mitigate the issue where both sides add the route
 	// on the opposite connection, and therefore end-up with both
 	// connections being dropped.