Remove unused index stats for MemoryTable
Concurrent insert+statistics test
indexIterator gets another boolean for snapshot
TableStatus#getApproximateRowCount gets Session paramter.

Use getApproximate when computing statistics.

Use snapshot read for FDB getApproximate.
Use snapshot range scan for index statistics traversal
merge nathanlws: Snapshot reads for statistics generation

Merge pull request #1264 from nathanlws/sql-layer/stats-are-snapshot
https://github.com/FoundationDB/sql-layer/pull/1264