Add monitor for inverted Indices

Add a class invertedIndexStatistics to store 1.the total number of docs indexed, 2.true number of docs we stored, 3.memory consumption and 4.index strategy for each column.

Update the computation of those statistics information.

Add indices monitor for invertedIndex, so that we can query in console with:
select com.senseidb.ba.monitor.IndexMonitorMapReduce(*)