ENG-6300 use EPHEMERAL_SEQUENTIAL zookeeper nodes as snapshot trauncation ids

Conflicts:
	src/frontend/org/voltdb/RestoreAgent.java
	src/frontend/org/voltdb/SnapshotDaemon.java
ENG-3900 fix backport compiler error
ENG-6300 use persistent sequential nodes for truncation snapshot requests
Merge pull request #1888 from VoltDB/ENG-6300-backport-to-4.0.1.x

ENG-6300 backport to 4.0.1.x