possibly fix deadlocking issues with tempo map by rearranging code and adding RT...

possibly fix deadlocking issues with tempo map by rearranging code and adding RT variants for a couple of functions that would normally try to rebuild the map when necessary; instead the variants throw an exception and the RT code that called them tries to do something reasonable

git-svn-id: svn://localhost/ardour2/branches/3.0@11164 d708f5d6-7413-0410-9779-e7cbd77b26cf