[Discovery] accumulated improvements to ZenDiscovery

This PR contains the accumulated work from the feautre/improve_zen branch. Here are the highlights of the changes: 

__Testing infra__
- Networking:
    - all symmetric partitioning
    - dropping packets
    - hard disconnects
    - Jepsen Tests
- Single node service disruptions:
    - Long GC / Halt
    - Slow cluster state updates
- Discovery settings
    - Easy to setup unicast with partial host list
	
__Zen Discovery__
- Pinging after master loss (no local elects) 
- Fixes the split brain issue: #2488
- Batching join requests
- More resilient joining process (wait on a publish from master)
I think it's OK? We want to prefer safety and copy the list anyway because we changing it's content. If that's the case might as well accept any Iterable. This code is rarely called so performance is secondary imho.