DERBY-6125 Code clean up in client driver

Patch derby-6125-04-a. Patch details:

- removed unused var Configuration.traceFileSuffixIndex__,
  #enableConnectivityToTargetServer__, #jvmSupportsMicrosClock

- made #dncCompatibleJREVersions private, added "1.8" as a legal JRE,
  and added accessor getDncCompatibleJREVersions

- made NetConnection#msgutil final

- removed unused var targetExtnam_

- made #indoubtTransactions_ private and changed to genericized
  HashMap, I believe this is multi thread safe, being used within a XA
  connection. Added accessors #getIndoubtTransaction and
  #getIndoubtTransactionIds

- made some methods final (e.g. #completeConnect, #flowConnect,
  #flowSimpleConnect, #isConnectionNull)

- removed unused extnam

- reduced some variable's scopes to silence FindBugs.

- made #targetPublicKey_ private, changed accessor #getTargetPublicKey
  to copy the the mutable array's contents

- removed instances of useless code of this shape:
      :
      if (!isXAConnection_)
           return;
      if (isOpen()) { // <--- has no side effects
           return; // still open, return
      }
  } <---- we return here anyway...

- removed unused vars NetConnectionReply#srvdgnReceived, #srvdgn

- replaced StringBuffer with StringBuilder in
  NetConnectionReply#doMgrlvlrmSemantics

- removed unused var NetXAResource#numXid

- removed unused var ClientBaseDataSourceRoot#globaltraceFileSuffixIndex

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1463080 13f79535-47bb-0310-9956-ffa450edef68