Conditionally increment target seq num on resend request

Created by: bhaan

fixes #169 (closed)

The cause of the referenced issue was due to the target seq num being incremented while both sides were sending the resend request. Therefore when they received the corresponding sequence reset, the sequence of that message was lower than what was erroneously expected. Looking at the quickfix/c++ code, there is in fact a check to prevent this from happening:

https://github.com/quickfix/quickfix/blob/6bc717f88f8e8c3729415a0ec65ec2615d501728/src/C%2B%2B/Session.cpp#L448-L449