Fix TENANT_LIMIT_SET_CALLS implementation

[jgnd]

The change introduced in #115 broke the TENANT_LIMIT_SET_CALLS because the DatabaseWrapper creates a new cursor every time and so the check self._previous_cursor != cursor is never false.
This behaviour is described in PEP 249:

.cursor()
Return a new Cursor Object using the connection.

and can be seen in the underlying implementation (in django.db.backends.base)

I have removed the check and also updated the tests to show the intended behaviour of the TENANT_LIMIT_SET_CALLS setting.

I realise this will probably recreate the issue described in #114 and I tried to figure out the underlying issue but without more information it is hard to diagnose.

[grjones]

It looks like this is only fixed in Django Tenants 2.0. I think it would be worth backporting this fix to 1.3.x since this is a critical fix, and Django 1.11 will be supported through 2020.

[domdinicola]

@tomturner any plan to release a version (actually 2, for django 1.x and 2.x) with this fix?

[tomturner]

Sorry I am not releasing any more version 1.x.