Do not collect deleted KVs when they are still in use by a scanner.

I noticed this because TestAtomicOperation.testMultiRowMutationMultiThreads fails rarely.
The solution is similar to HBASE-2856, where expired KVs are not collected when in use by a scanner.

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What I pieced together so far is that it is the scanning side that has problems sometimes.

Every time I see a assertion failure in the log I see this before:

2012-03-12 21:48:49,523 DEBUG [Thread-211] regionserver.StoreScanner(499): Storescanner.peek() is changed where before = rowB/colfamily11:qual1/75366/Put/vlen=6,and after = rowB/colfamily11:qual1/75203/DeleteColumn/vlen=0

The order of if the Put and Delete is sometimes reversed.

The test threads should always see exactly one KV, if the "before" was the Put the thread see 0 KVs, if the "before" was the Delete the threads see 2 KVs.

This debug message comes from StoreScanner to checkReseek. It seems we still some consistency issue with scanning sometimes