Replace Traversable with Visitable, simplify Visitor
merge nathanlws: Replace Traversable with Visitable, simplify Visitor

Merge pull request #1014 from nathanlws/sql-layer/traversable-to-visitable
https://github.com/FoundationDB/sql-layer/pull/1014
Track a version number for the generated group.
Persist generated storage format descriptors.
Add conversion for data as well as meta-data.
Add format for Protobuf in Persistit.
Add a unit test.