deadlock when disconnecting dynamic session

this kind of writing in channel makes deadlock possibility! https://github.com/quickfixgo/quickfix/blob/7945e4b53d7be2e68202fe68952ee31f2a736ccc/session.go#L742

Possible deadlock occurs in dynamic session closing because line https://github.com/quickfixgo/quickfix/blob/7945e4b53d7be2e68202fe68952ee31f2a736ccc/session.go#L768

is the only reader from channel s.sessionEvent, and after ending of loop https://github.com/quickfixgo/quickfix/blob/7945e4b53d7be2e68202fe68952ee31f2a736ccc/session.go#L752 no one can read from channel s.sessionEvent

PR #523

Thanks!