Removed unneccesary PGThread.

Django connections are thread locals by default, it's safe to attach
attributes to the DatabaseWrapper, because those are thread safe.
Removed connection.get_schema() and connection.get_tenant() methods,
those were not very pythonic at all. Use connection.schema_name and
connection.tenant properties instead.
Resolved merge conflicts on docs regarding tenant and schema_name
readded backwards compatibility (get_schema_name and get_tenant)