PYTHON-785 Delete TestReplicaSetRequest.
PYTHON-785 Don't call start_request in mod_wsgi test.
PYTHON-785 Remove start_request().

Remove MongoClient's methods start_request(), in_request(), and end_request().
The purpose of requests was to provide read-your-writes consistency when using
w=0 write concern. Starting a request pins a socket to a thread so any
operations on that thread end up in the same queue on the server side.

Justification for removing: mongos 2.6+ doesn't support socket pinning by
default, and mongos 2.8+ doesn't support it at all (SERVER-12273), so whatever
weak consistency guarantees a request was supposed to provide are not provided
with sharding.

It's unnecessary with MongoDB 2.6+ since write commands always block and send a
response.

It's a confusing feature that should rarely be used, if ever, yet people
incorrectly use it all the time. It makes our connection pool unmaintainable to
any but the most expert developers.
PYTHON-785 Delete examples/requests.rst.