remove recursive mutex from Playlist, replace with private regular mutex, force every...

remove recursive mutex from Playlist, replace with private regular mutex, force everyone to use Playlist::RegionLock to allow checking on lock handling if necessary; fix recursive use of lock in AudioPlaylist::read()

git-svn-id: svn://localhost/ardour2/branches/3.0@12131 d708f5d6-7413-0410-9779-e7cbd77b26cf