fix two major assert failures arising from the optional monitor section commit; separ...

fix two major assert failures arising from the optional monitor section commit; separate numbering of aux sends, sends and listens to fix #3671 (still testing, but the assert failures are critical)

git-svn-id: svn://localhost/ardour2/branches/3.0@11263 d708f5d6-7413-0410-9779-e7cbd77b26cf