SessionState.Store (FileStore) is not threadsafe on .Set and .Get

During a resendrequest - a .Get is done on the SessionState.Store.  This
get is not synchronized with .Set calls done on sending messages.  If
sending is initiated concurrently garbled or incorrect message data can
be resent.
Merge branch 'master' of github.com:jacsuper/quickfixn into jacsuper
credits for #258
name correction
Merge pull request #261 from connamara/jacsuper

SessionState.Store (FileStore) is not threadsafe on .Set and .Get