SQLExecutor is a new class intended for use whenever SQL statements

need to be executed. It's responsible for transaction management, SQL

statement preparation, and execution, and replaces the old run_sql(),

write_sql(), and execute_sql() functions.
The main thing this brings to the table is improved transaction

management. Normally with Django, you either use low-level transaction

commands (such as transaction.commit()) and juggle the various error

handling and savepoint situations yourself, or you make use of

transaction.atomic() to manage all this.
The problem with transaction.atomic() is that, being a context

manager, you can't easily start one, let it sit around outside of that

context, and then later stop it without some ugly code.
SQLExecutor simplifies this by turning transaction management and SQL

executions into just separate instructions that can be executed in

sequence. A caller can start a transaction, execute SQL, start a new one

(which will commit the previous), execute more SQL, commit, execute new

SQL outside a transaction, etc.
This change lays the ground work for this. The next change will allow

generated SQL to be marked as requiring an independent transaction or

requiring being run outside of a transaction, which will be factored in

during the SQL preparation step.

Unit tests passed.
Tested creating and upgrading databases successfully.