A concurrent linked list, to replace the array list of old roots (work in progress)

git-svn-id: http://h2database.googlecode.com/svn/trunk@5779 e6896862-9d19-0410-bfb3-e9fa5d50e656
Issue 567: h2 hangs for a long time then (sometimes) recovers.
Introduce a queue when doing table locking to prevent session starvation.


git-svn-id: http://h2database.googlecode.com/svn/trunk@5780 e6896862-9d19-0410-bfb3-e9fa5d50e656