ENG-5511: Overload HostMessenger.sendPoisonPill() to take hostIds.
ENG-5511: Continue deleting ZK nodes even if a node doesn't exist in
ZKUtils.deleteRecursively().

Some nodes may disappear while deleteRecursively() is walking the ZK nodes,
ignore NoNodeException since they are harmless.
Merge pull request #951 from VoltDB/ENG-5511

ENG-5511: Refactoring in HostMessenger and ZKUtils