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

This pull request contains the following changes:

1) pool manager now has instance variables, max_bson_size and max_message_size that are set to the default values for both on initialization of a pool manager.

2) Changes to a pool manager's max_bson_size and max_message_size variables are now done via "push" messages rather than "pulling".  A node sends a message to its pool manager to update its max_bson_size and max_message_size whenever its "set_config" method is called.  The manager will get the min amongst all known members (i.e. nodes).