unnecessary database hits in model_factory.py and collector.py

[fariddarabi]

There is some unnecessary database hits in some modules.

first, we set request.prof_file and immediatelly call method save() on it, but it's not necessary. because method _process_response in the middleware will call save() at the end.

path: collector.finalise

again, method save() is called on request after meta_num_queries and meta_time_spent_queries is set.
unnecessary, because the request object will be saved in the middleware in _process_response

path: collector._record_meta_profiling

in ResponseModelFactory.construct_response_model, a Response object is created by method create(), so it is immediatelly saved to the database. next, Response.raw_body is set and method save() is called in the middleware.
database hits could be decreased

model_factory.ResponseModelFactory.construct_response_model

SilkyMiddleware._process_response

[albertyw]

Fixed in #588