remove some dead semicolons and fix a javadoc

git-svn-id: http://h2database.googlecode.com/svn/trunk@5284 e6896862-9d19-0410-bfb3-e9fa5d50e656
simplify background writer thread implementation, tying the sync object lifetime more closely to the thread object lifetime

git-svn-id: http://h2database.googlecode.com/svn/trunk@5285 e6896862-9d19-0410-bfb3-e9fa5d50e656