Consistent null adapter handling
Accessor for expressions
Lookup by table ID
Build an AIS-cached set of transforms for ALTER
Constraint checking goes through transform cache.
Index building goes through transform
OnlineHelper gets instantiated
Cleanup
merge nathanlws: OnlineHelper caches transformation info

Merge pull request #1074 from nathanlws/sql-layer/online-cached-transforms
https://github.com/FoundationDB/sql-layer/pull/1074