More load balancing policies

[tsegismont]

In addition to the existing (round-robin and least request) policies in io.vertx.core.loadbalancing.LoadBalancer, we should have:

• random
• power of two choices

Possible contributor @machine424

[tsegismont]

@vietj can you elaborate about the hashing load balancing policies you talked to me about off-list? I understand this could be used to solve different cases, including sticky web sessions

[vietj]

this is used indeed for sticky session

this policy requires API modification because we need to pass this ID from the request (HTTP but not necessarily) to the load balancing algorithm.

most likely EndpointResolver#lookupEndpoint(ContextInternal,A) needs to have this ID as parameter.

Users would use it using RequestOptions for HTTP requests and optionally set it.