Mongos load balancing (PYTHON-852) introduces a race related to MongoClient._writable_max_wire_version. If I start a sharded cluster with a 2.4 and a 2.6 mongos and connect to both, half the time _writable_max_wire_version is 2, half the time it is zero.

After a small random number of iterations this code:

for i in range(1000):
    client = MongoClient('mongodb://localhost:1026,localhost:1027')
    client.db.collection.insert({})
    print i

... attempts a 2.6-style write command against the 2.4 mongos and throws:

Traceback (most recent call last):
  File "writable.py", line 6, in <module>
    client.db.collection.insert({})
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/collection.py", line 1921, in insert
    check_keys, manipulate, write_concern)
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/collection.py", line 411, in _insert
    gen(), check_keys, self.codec_options, client)
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/message.py", line 435, in _do_batched_write_command
    results.append((idx_offset, send_message()))
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/message.py", line 387, in send_message
    command=True)
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/mongo_client.py", line 804, in _send_message
    return self.__check_gle_response(response.data, command)
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/mongo_client.py", line 727, in __check_gle_response
    helpers._check_command_response(result)
  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/helpers.py", line 179, in _check_command_response
    raise OperationFailure(msg % errmsg, code, response)
pymongo.errors.OperationFailure: no such cmd: insert

We can use this opportunity to remove races related to outdated ServerDescriptions, and rely on each socket's ismaster response (PYTHON-829):

• *DONE*: Remove MongoClient._writable_max_wire_version().
• *DONE*: Methods that used it must select a server and call get_socket() for a SocketInfo, and use that SocketInfo's wire version (which came from an ismaster call on that socket when it was opened) to determine the wire protocol, before formatting a message.
• *DONE*: Functions in message.py (and _cmessagemodule) that took a client instance take SocketInfo instead.
• *DONE*: Add SocketInfo.send_message_with_response and other APIs that message.py needs.
• *DONE*: Move Server._check_bson_size to SocketInfo, ensure it's tested for all paths.
• *DONE*: Refactor SocketInfo.command, helpers._command, and network.command?
• *DONE*: Delete MongoClient._send_message?
• *DONE*: Actually retest against live mixed version cluster, and add test to pymongo-mockup-tests
• *DONE*: For absolute correctness in Collection.options(), and Database.collection_names() we should get a socket, check its wire version, and if it's < 3 use that socket to query the system collection
• *DONE*: Delete test_wire_version_mongos_ha, which should have been deleted in PYTHON-852 anyway.
• *DONE*: Delete max_wire_version etc. from MongoClient