MVTableEngine: support online backup on Windows

git-svn-id: http://h2database.googlecode.com/svn/trunk@4948 e6896862-9d19-0410-bfb3-e9fa5d50e656
Test: avoid OOME when testing in-memory databases.

git-svn-id: http://h2database.googlecode.com/svn/trunk@4949 e6896862-9d19-0410-bfb3-e9fa5d50e656