Decouple query generator from execution

[darkain]

Decouple the PUDL query generator from the PUDL execution unit.

This task will not be done for quite some time, this is a post 3.0 task.

PDO, ODBC, and the various execution proxies can all support a wide variety of SQL servers. Having the query generator for a specific engine tied to a specific execution unit means that potential options for users are being locked out.

The main standard API and instantiation for MySQL, PostgreSQL, SQL Server, and SQLite will remain unchanged. Each of these execution units will default to their proper query generators.

pudlNull will accept any query generator for debug purposes.

PDO, ODBC, and Proxies will have an additional (required?) parameter to specify which generator they wish to use.

[darkain]

Execution unit and connection manager should also be split, so any connection type can inherently support clustering and read/write splitting.