Unauthenticated Logon can reset FIX session store

[imirkin]

Let's say you have an application which performs verification of Logons in FromAdmin and returns an appropriate RejectLogon error when authentication fails. When a client sends a Logon with ResetSeqNumFlag=Y without the proper authentication, session::handleLogon will reset the store anyways. (Let me know if you need further commentary as to why this is bad.)