Cleanup findbug warnings in branch-3.4: Dodgy code Warnings

DB	org.apache.zookeeper.server.quorum.auth.SaslQuorumAuthLearner.send(DataOutputStream, byte[]) uses the same code for two branches
DLS	Dead store to txn in org.apache.zookeeper.server.quorum.LearnerHandler.packetToString(QuorumPacket)
NP	Load of known null value in org.apache.zookeeper.server.PrepRequestProcessor.pRequest(Request)
NP	Possible null pointer dereference in org.apache.zookeeper.server.PurgeTxnLog.purgeOlderSnapshots(FileTxnSnapLog, File) due to return value of called method
NP	Possible null pointer dereference in org.apache.zookeeper.server.PurgeTxnLog.purgeOlderSnapshots(FileTxnSnapLog, File) due to return value of called method
NP	Load of known null value in org.apache.zookeeper.server.quorum.auth.SaslQuorumAuthLearner.send(DataOutputStream, byte[])
NP	Load of known null value in org.apache.zookeeper.server.quorum.auth.SaslQuorumAuthServer.send(DataOutputStream, byte[], QuorumAuth$Status)
NP	Possible null pointer dereference in org.apache.zookeeper.server.upgrade.UpgradeMain.copyFiles(File, File, String) due to return value of called method
RCN	Redundant nullcheck of bytes, which is known to be non-null in org.apache.zookeeper.server.persistence.FileTxnLog$FileTxnIterator.next()
SF	Switch statement found in org.apache.zookeeper.server.PrepRequestProcessor.pRequest(Request) where default case is missing
SF	Switch statement found in org.apache.zookeeper.server.PrepRequestProcessor.pRequest2Txn(int, long, Request, Record, boolean) where default case is missing
SF	Switch statement found in org.apache.zookeeper.server.quorum.AuthFastLeaderElection$Messenger$WorkerReceiver.run() where default case is missing
SF	Switch statement found in org.apache.zookeeper.server.quorum.AuthFastLeaderElection$Messenger$WorkerSender.process(AuthFastLeaderElection$ToSend) where default case is missing
SF	Switch statement found in org.apache.zookeeper.server.quorum.Follower.processPacket(QuorumPacket) where default case is missing
SF	Switch statement found in org.apache.zookeeper.server.quorum.Observer.processPacket(QuorumPacket) where default case is missing
ST	Write to static field org.apache.zookeeper.server.SyncRequestProcessor.randRoll from instance method org.apache.zookeeper.server.SyncRequestProcessor.run()
UrF	Unread public/protected field: org.apache.zookeeper.server.upgrade.DataTreeV1$ProcessTxnResult.err
UrF	Unread public/protected field: org.apache.zookeeper.server.upgrade.DataTreeV1$ProcessTxnResult.path
UrF	Unread public/protected field: org.apache.zookeeper.server.upgrade.DataTreeV1$ProcessTxnResult.stat
UrF	Unread public/protected field: org.apache.zookeeper.server.upgrade.DataTreeV1$ProcessTxnResult.type