Improved test coverage

git-svn-id: http://h2database.googlecode.com/svn/trunk@5234 e6896862-9d19-0410-bfb3-e9fa5d50e656
MVStore: use a write buffer instead of a ByteBuffer

git-svn-id: http://h2database.googlecode.com/svn/trunk@5235 e6896862-9d19-0410-bfb3-e9fa5d50e656