MVStore: closing a map and making it read-only is now only a marker and not enforced, to allow concurrent store operations

git-svn-id: http://h2database.googlecode.com/svn/trunk@5249 e6896862-9d19-0410-bfb3-e9fa5d50e656
Documentation

git-svn-id: http://h2database.googlecode.com/svn/trunk@5250 e6896862-9d19-0410-bfb3-e9fa5d50e656