We have a number of optimizations in place for Django unit test

fixtures, designed to prevent unnecessary repeated deserialization of

fixture data. This change improves this further, reducing the work

needed to save the deserialized data to the database.
Previously, we'd call save() on every deserialized object for every

test run. This would perform an UPDATE call (to update a previous

database instance with the object's ID, to reflect the new state),

falling back to a new INSERT if the UPDATE didn't find any existing

rows. This meant, in the worst-case scenario, 2 SQL queries per fixture

object per unit test.
We now simply delete any existing entries all at once (if any exist) and

then proceed to bulk-create the replacements. This saves us all those

SQL queries, requiring only 2 per model instead of 2 per object per

model.
On top of this, we no longer directly assign a list of objects to

ManyToManyFields. An assignment results in a SELECT, possible

DELETE, and an INSERT, and this is wasteful when we know we have new

entries. So instead, we call .add() on the relation so that only an

INSERT is done.
In testing against the Review Board unit test suite (of ~4000 unit

tests), this change resulted in a ~45 second reduction in test time with

no regressions introduced.

Djblets and Review Board unit tests pass.