Implement ECMP for unsafe_routes

[dioss-Machiel]

This PR implements ECMP support in Nebula. The implementation uses hash-threshold mapping (like in the Linux kernel) which allows you to define weights per gateway.

This can be used for example to aggregate multiple links and provide redundancy to an external location where nebula cannot be installed.

The change is backwards compatible and should not impact normal operation.
ECMP routes can be defined in the config file (example below) and also via the use_system_route_table feature, which means that on Linux you can use a BGP daemon to add / remove multipath routes.

Example config:

tun:
  unsafe_routes:
	# Backwards compatibility
	- route: 192.168.86.0/24
	  via: 192.168.100.10
	
	# Single gateway, weight optional
	- route: 192.168.87.0/24
	  via: 
		- gateway: 10.0.0.1

	# Multipath, weight optional (defaults to 1), will balance equally
	- route: 192.168.87.0/24
	  via: 
		- gateway: 10.0.0.1
		- gateway: 10.0.0.2

	# Multipath, weight defined, will balance according to weights
	- route: 192.168.87.0/24
	  via: 
		- gateway: 10.0.0.1
		  weight: 10
		  
		- gateway: 10.0.0.2
		  weight: 5

Implementation note: if the gateway where the packet should be routed to is not reachable the first available gateway is chosen, so when one of the gateways is down the traffic is no longer properly being balanced, but there is still connectivity.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @dioss-Machiel to sign the Salesforce Inc. Contributor License Agreement.