Added SocketSendBufferSize and SocketReceiveBufferSize settings.

Added settings to set the socket send and receive buffer sizes. These settings were available in quickfixj and we used them, so I added them migrating to quickfixn (I was surprised to find that they are not available in the "standard" quickfix version, at least not according to http://www.quickfixengine.org/quickfix/doc/html/configuration.html .)