RUBY-568 Change to max message and max bson size setters

This pull request contains the following changes:

1) The max_bson_size and max_message_size methods on a client no longer consult the local_manager to determine their values.

2) @max_bson_size and @max_message_size are set to their default values when there are no members available on the local_manager of a replica set client.

3) @max_bson_size and @max_message_size are set to the value in the config of the first of local_manager's members.  (possible alternate solution: use the primary's config)

4) There is an option on a client to force it to always use a particular max_message_size and max_bson_size.  This should be used when it is anticipated that there will be different versions of mongodb in a RS, potentially with different values in their config for maxBsonObjectSize and maxMessageSizeBytes.