Better connection pooling

To eliminate socket-per-thread high connection issues

• automatically share sockets between threads by default
• restore start_request to make thread local sockets optional
• consider mgo's monotonic consistency option for requests?
• make implementing PYTHON-296 and PYTHON-297 easier

I'm going to remove pymongo.connection._Pool and have all Connection classes (single-server, master-slave, replica-set) use the same pymongo.pool.Pool class.