Remove the unused {Insert,Update,Delete}_Default operators.

Along with SelectionFunction, as nowhere is actually calling
rowIsSelected.
merge nathanlws: Remove the unused {Insert,Update,Delete}_Default operators

Merge pull request #1516 from nathanlws/sql-layer/remove-old-iud-operators
https://github.com/FoundationDB/sql-layer/pull/1516
Change parts of FDB services from private to protected to allow specialization for testing.
Introduce a separate method for the top-level transaction context, defaulting to the database itself.