Long record pruning broken in stress tests

Akiban-persistit rev 311

Errors like this result from running the mixture_txn_1 and stress8_txn stress tests plans:

Debug assert1
    java.lang.RuntimeException
     at com.persistit.util.Debug.logDebugMessage(Debug.java:97)
     at com.persistit.util.Debug.access$000(Debug.java:36)
     at com.persistit.util.Debug$Assert.t(Debug.java:64)
     at com.persistit.Value.changeLongRecordMode(Value.java:4086)
     at com.persistit.LongRecordHelper.storeLongRecord(LongRecordHelper.java:231)
     at com.persistit.Exchange.storeInternal(Exchange.java:1530)
     at com.persistit.Exchange.store(Exchange.java:1294)
     at com.persistit.Exchange.store(Exchange.java:2534)
     at com.persistit.stress.Stress2txn$1.runTransaction(Stress2txn.java:147)
     at com.persistit.Transaction.run(Transaction.java:970)
     at com.persistit.stress.Stress2txn.executeTest(Stress2txn.java:144)
     at com.persistit.test.AbstractTestRunnerItem.runIt(AbstractTestRunnerItem.java:156)
     at com.persistit.test.TestRunner$TestThread.run(TestRunner.java:361)

FAILED WITH EXCEPTION
java.lang.IllegalArgumentException: Bad LONG_RECORD descriptor size: 3171
 at com.persistit.Buffer.writeLongRecordDescriptor(Buffer.java:3278)
 at com.persistit.LongRecordHelper.storeLongRecord(LongRecordHelper.java:219)
 at com.persistit.Exchange.storeInternal(Exchange.java:1530)
 at com.persistit.Exchange.store(Exchange.java:1294)
 at com.persistit.Exchange.store(Exchange.java:2534)
 at com.persistit.stress.Stress2txn$1.runTransaction(Stress2txn.java:147)
 at com.persistit.Transaction.run(Transaction.java:970)
 at com.persistit.stress.Stress2txn.executeTest(Stress2txn.java:144)
 at com.persistit.test.AbstractTestRunnerItem.runIt(AbstractTestRunnerItem.java:156)
 at com.persistit.test.TestRunner$TestThread.run(TestRunner.java:361)

Undoubtedly this bug was created by the new pruneLongMVV code in Buffer.