Create UndoLog only when necessary and remove outdated code

Remove some PageStore+MVCC leftovers

Fix inconsistencies in checks for transaction isolation level

Fix issues with testing on latest Java versions

# Conflicts:
#	h2/src/main/org/h2/mvstore/tx/TxDecisionMaker.java

Leftovers handling

Fix for issue #1220

merge

Postpone session.endStatement() until after commit.

Test utilities only once during TestAll and fix OOME in TestOptimizations

Exception handling: preserve original exception.
Minor re-factoring

KillRestart fix