Plugins: Remove pluggability of ZenPing
ZenPing is the part of zen discovery which knows how to ping nodes.
There is only one alternative implementation, which is just for testing.
This change removes the ability to add custom zen pings, and instead
hooks in the MockZenPing for tests through an overridden method in
MockNode. This also folds in the ZenPingService (which was really just a
single method) into ZenDiscovery, and removes the idea of having
multiple ZenPing instances. Finally, this was the last usage of the
ExtensionPoint classes, so that is also removed here.
Clinton Gormley
Ryan Ernst
Ali Beyad