Update the FDBStoreIndexStatistics so it gets a snapshot read when
getting the IndexStatisistics from the underlying storage layer. Avoids
a past version error when the index generator overwrites the earlier
read from the same table.
Now that histogram reads are snapshots, you can't write and read them in
the same transaction. Fix the tests by splitting the write/read cycle
into two transactions. Everything passes again.
Add a missing @Override to the
ColumnKeysStorageDescription#groupIterator, plus affected callers, to
address test failure caused by the groupIterator not retriving data
correctly.
merge tjoneslo: Index statistics snapshot read

Merge pull request #1354 from tjoneslo/sql-layer/Index-statistics-snapshot-read
https://github.com/FoundationDB/sql-layer/pull/1354