[#304] Throw PersistitClosedException when a commit races Persistit.close()

[vharseko]

Fixes #304

Problem

A transaction commit racing Persistit.close() has two failure windows in JournalManager.waitForDurability:

1. Raw IllegalStateException: close() nulls the _flusher reference before the committing thread reads it, so waitForDurability throws IllegalStateException("JOURNAL_FLUSHER is not running"). That is not a PersistitException, so callers' closed-state handling is bypassed — observed as the TransactionTest2.transactionsConcurrentWithPersistitClose flake on master run 30078532797, where the worker landed in the test's catch (Throwable) branch and the stranded-thread assert failed.
2. Infinite poll (latent): a committing thread that captures the flusher reference just before it is cleared enters the JournalFlusher.waitForDurability polling loop; once the flusher stops, its durability marks never advance, and a flushedTimestamp allocated after the flusher's final cycle can never be covered — the loop kicks a stopped thread forever. Confirmed real: the new test hangs past a 10-second join deadline without the fix.

Fix

Throw PersistitClosedException in both windows:

• JournalManager.waitForDurability throws PersistitClosedException instead of IllegalStateException when the flusher reference is already null, so a commit racing close() unwinds through the same documented path as every other closed-state operation.
• The JournalFlusher.waitForDurability polling loop checks isStopped() after the success check. The flusher publishes its final _startTimestamp/_endTimestamp before stopping, so a single re-read after observing the stop is exact: either the final flush cycle covered the caller's timestamp (return success) or no future cycle ever will (throw PersistitClosedException).

TransactionTest2 needs no changes: PersistitClosedException is already one of its expected shutdown outcomes.

Testing

• Two new JournalManagerTest cases, one per window: waitForDurabilityAfterCloseThrowsPersistitClosedException (direct call after close()) and waitForDurabilityOnStoppedFlusherThrowsInsteadOfSpinning (via crash(), which stops the flusher without clearing the reference). Both verified to fail without the fix — the first with the raw IllegalStateException, the second by hanging past its join deadline.
• Full persistit/core suite on JDK 11: 592 tests, 0 failures, 0 errors, 5 skipped.