* Rename BaseQuerySet._initial_query to BaseQuerySet._cls_query
This new name more accurately reflects the purpose of the dict. It is either
empty for documents that don't use inheritance or it contains a `{"_cls": ...}`
shape query. There was nothing "initial" about it per se.
* Drop read_preference as a kwarg on BaseQuerySet.__call__/filter
It was a poor design choice to offer two different ways to do the same thing:
1. `SomeDoc.objects(foo=bar, bar=baz).read_preference(...)`
2. `SomeDoc.objects(foo=bar, bar=baz, read_preference=...)`
Option 1 is good because it's immediately obvious which part defines the query
to be used and which part defines the read preference.
Option 2 is bad because you don't immediately know whether `read_preference`
is a query kwarg or a reserved keyword with some special behavior. If you
wanted to be particularly cruel, you could even write
`SomeDoc.objects(foo=bar, read_preference=..., bar=baz)`.
THIS IS A BREAKING CHANGE. From now on you have to use the
`BaseQuerySet.read_preference(...)` method.
* Add a BaseQuerySet.clear_cls_query method + get rid of the class_check kwarg
This is similar to what the previous commit did to read preference except that
in this case we were still missing a `BaseQuerySet` method for clearing the
`_cls` query.
Now, instead of the undocumented, untested, and confusing interface:
`ParentDoc.objects(foo=bar, bar=baz, class_check=False)`
We do:
`ParentDoc.objects(foo=bar, bar=baz).clear_cls_query()`
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