MVTableEngine: make problems of the writer thread visible

git-svn-id: http://h2database.googlecode.com/svn/trunk@4960 e6896862-9d19-0410-bfb3-e9fa5d50e656
Avoid concurrency problems of the freedPages map

git-svn-id: http://h2database.googlecode.com/svn/trunk@4961 e6896862-9d19-0410-bfb3-e9fa5d50e656
MVRTreeMap: never free a page twice

git-svn-id: http://h2database.googlecode.com/svn/trunk@4962 e6896862-9d19-0410-bfb3-e9fa5d50e656