remove txn attribute from DB

txn attribute was never actually used

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>
ConstantTimeBackoff test and cleanup

- javadoc changed - the old one was copy-pasted from AgentShell
- start and stop method removed - they did the same as the overridden methods
- _counter removed as it was only written, but never read
- remove from _asleep map was moved to a finally block, to make sure it is removed even in case of the thread gets interrupted
- Tests created for the above scenarios.

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>