HBASE-694 HStore.rowAtOrBeforeFromMapFile() fails to locate the row if # of mapfiles >= 2
-Added new test to TestGet2 to highlight multi-storefile getClosestBefore issue
-Removed erroneous return that caused 2nd and subsequent mapfiles to be skipped
-Split HStore#rowKeyAtOrBeforeFromMapfile into two sub-methods for readability
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