Lack of session's message store interface

[lacombar]

QuickFIX/J's Session objects have a getStore() accessor lacking in the Golang implementation. cf. http://quickfixengine.org/quickfix/doc/html/class_f_i_x_1_1_session.html#a1f3aa9ad3152de9ed6f09223d309977a

While not ideal, some exchanges have really finicky sequence number management and having direct access to a session's store would help greatly.