Revision 651017

HBASE-588 Still a 'hole' in scanners, even after HBASE-532

Add a ChangedReadersObserver interface.  HStore notifies registered
observers when list of HStoreFile Readers changes -- at flush time
and at compaction time.  Scanners are only current observers.

Also fix a deadlock flushing by changing lock types and moving flush
request out from under lock.

M  src/test/org/apache/hadoop/hbase/MultiRegionTable.java
    (getCacheFlushListern): Renamed as getFlushRequester
M  src/java/org/apache/hadoop/hbase/regionserver/HStore.java
    Added synchronizations on this.storefiles.
    (activeScanners, newScannerLock): Removed.  We no longer try to
    block out scanners when compacting (Turns out same functionality
    for blocking scanners is up in HRegion used around closing/split).
    (changedReadersObservers): Added.
    (updateReaders): New method that manages the insertion of new
    reader on flush.  Also calls new notifyChangedReadersObservers.
    (notifyChagnedReadersObservers, addChangedReadersObserver,
      deleteChangedReadersObservers): Added.
    (completeCompaction): Previous deleting old store files, we'd
    remove from this.storefiles and delete all in one step; now we
    do the remove first, notify all observers of readers, and then
    do the delete so observers have a chance to clean up any old
    references to files about to be deleted.  Removed all the lockout
    of new scanner creation and wait on old scanners to come int.
    (updateActiveScanners): Removed.
    (getStorefiles): Accessor.  Added.
M  src/java/org/apache/hadoop/hbase/regionserver/StoreFileScanner.java
    Added implementation of new ChangedReadersObserver interface.
    Added a lock that we hold when 'nexting' and when changing the
    set of readers out from under the scanner.
    Changed the constructor moving bulk into new openReaders method
    that we reuse when list of Readers changes.
    (next): Moved head of this method into new getNextViableRow (used
    to be called chosenRow and chosenTimestamp).  New method returns
    simple datastructure of row and timestamp (ViableRow).
    (close): On close, remove ourselves as ChangedReadersObserver (we
    added ourselves in the constructor).
    (updateReaders): Changed the set of Readers out from under the
    Scanner.
A  src/java/org/apache/hadoop/hbase/regionserver/ChangedReadersObserver.java
    Added.
M  src/java/org/apache/hadoop/hbase/regionserver/Flusher.java
    Changed name of the interface we implement from CacheFlushListener to
    FlushRequester.
D  src/java/org/apache/hadoop/hbase/regionserver/CacheFlushListener.java
    Renamed as FlushRequester.
M  src/java/org/apache/hadoop/hbase/regionserver/HStoreScanner.java
    Remove update of activeScanners.
A  src/java/org/apache/hadoop/hbase/regionserver/FlushRequester.java
    Added.  Rename of CacheFlushListener.
M  src/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java
    Renamed method getCacheFlushListener as getFlushRequester.
M src/java/org/apache/hadoop/hbase/regionserver/HAbstractScanner.java
    Formatting.
M  src/java/org/apache/hadoop/hbase/regionserver/HRegion.java
    CacheFlushListener was renamed as FlushListener.