Eliminate window of missing Data

After incorporating v11 of the 2856 fix, we discovered that we are still having some ACID violations.

This time, however, the problem is not about including "newer" updates; but, about missing older updates
that should be including.

Here is what seems to be happening.

There is a race condition in the StoreScanner.getScanners()

private List<KeyValueScanner> getScanners(Scan scan,
final NavigableSet<byte[]> columns) throws IOException {
// First the store file scanners
List<StoreFileScanner> sfScanners = StoreFileScanner
.getScannersForStoreFiles(store.getStorefiles(), cacheBlocks,
isGet, false);
List<KeyValueScanner> scanners =
new ArrayList<KeyValueScanner>(sfScanners.size()+1);

// include only those scan files which pass all filters
for (StoreFileScanner sfs : sfScanners) {
if (sfs.shouldSeek(scan, columns))

{ scanners.add(sfs); }

}

// Then the memstore scanners
if (this.store.memstore.shouldSeek(scan))

{ scanners.addAll(this.store.memstore.getScanners()); }

return scanners;
}

If for example there is a call to Store.updateStorefiles() that happens between
the store.getStorefiles() and this.store.memstore.getScanners(); then
it is possible that there was a new HFile created, that is not seen by the
StoreScanner, and the data is not present in the Memstore.snapshot either.