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.