Extenal tree name addition for NameGenerator
Don't double generate PK tree names during ALTER
New methods on SchemaManager for derived flexibility
New method on Store. Fix TABLE and GROUP ALTER locking.
merge nathanlws: SchemaManager and Store flexibility

Merge pull request #901 from nathanlws/sql-layer/schema-manager-flexibility
https://github.com/FoundationDB/sql-layer/pull/901