Support for disabling concurrent DML via config
Turn writes on for tests and ensure disabling works
merge nathanlws: Disable concurrent DML by default

Merge pull request #1086 from nathanlws/sql-layer/concurrent-dml-config
https://github.com/FoundationDB/sql-layer/pull/1086
Enough of the basic data structure to get from CREATE TABLE to AIS (via Protobuf).
Some ALTER support.
Emulated meta-data.
Clean up error handling.
I_S tables.
Add a YAML test for those DDL operations.
Sketch the overall flow.
Clarify multi-column conditions and add a test.