IndexMeta removal prevents index class reuse

[giovannibelzoni]

#998

This PR has made index class reuse impossible, due in part by the __set_name__ method on the index class setting the first model it's used on.

Prior to this change, it was possible to define an index class and use it on multiple models. Now we see:

In [2]: ModelA._indexes['MyIndex'].Meta.model
Out[2]: app.models.ModelA

In [3]: ModelB._indexes['MyIndex'].Meta.model
Out[3]: app.models.ModelA

I suspect this is the problem for this reported bug:
#1026

[ikonst]

We could allow reuse since an index should only be useable as an instance on a model, i.e. MyIndex.query(...) should no longer be possible in the current version.