only create a CIDRTree for each host if necessary (#198)

A CIDRTree can be expensive to create, so only do it if we need
it. If the remote host only has one IP address and no subnets, just do
an exact IP match instead.

Fixes: #171

hostmap.go

@@ -623,6 +623,11 @@ func (i *HostInfo) RecvErrorExceeded() bool {
 }
 
 func (i *HostInfo) CreateRemoteCIDR(c *cert.NebulaCertificate) {
+	if len(c.Details.Ips) == 1 && len(c.Details.Subnets) == 0 {
+		// Simple case, no CIDRTree needed
+		return
+	}
+
 	remoteCidr := NewCIDRTree()
 	for _, ip := range c.Details.Ips {
 		remoteCidr.AddCIDR(&net.IPNet{IP: ip.IP, Mask: net.IPMask{255, 255, 255, 255}}, struct{}{})