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Fix error in batch scanning, which led to a unittest failure in random
runs for MVCC stats.
Merge pull request #123 from cockroachdb/spencerkimball/fix-random-runs

Fix error in batch scanning, which led to a unittest failure in random
Replace Engine.Scan with Engine.Iterate

Move to iteration-based primitive for efficiency. This is going to be
particularly useful when we start to optimize mvcc.Scan, which
currently is very, very inefficient.

Scan and ScanSnapshot are still provided as convenience functions
which take an Engine instance as a parameter. Most existing code
has been moved to use those. Other bits of code have been moved
to use the new iteration-based approach.

Fixed a bug in Batch.Scan where insufficient entries would be
returned in the event a deletion in the updates map overrode
a value in the underlying engine and the max scan parameter was
exceeded.
Flush MVCC stats to range and store counters.

On successful execution of a range read-write command, flush accumulated
mvcc stats to merging counters for range and store.
Address Ben/Tobias feedback
Address feedback comments
Merge pull request #124 from cockroachdb/spencerkimball/fix-random-runs

Replace Engine.Scan with Engine.Iterate
Merge pull request #125 from cockroachdb/spencerkimball/stat-counters

Flush MVCC stats to range and store counters.
deal with clock uncertainty for transactional readers
address Spencer's comments