Improve the error raised by too much contention on a sequence/identity.

Currently, when there is too much contention on a sequence/identity, Derby raises an error saying so. There are two properties which the user can adjust in order to reduce the risk of this error:

derby.locks.waitTimeout
derby.language.sequence.preallocator

It would be good to point the user at these knobs. The following change would improve this error reporting:

1) Raise a lock timeout SQLException

2) Chain a "too much contention" SQLException (X0Y84) to the lock timeout.

3) Make the "too much contention" exception suggest that the user adjust the properties mentioned above.

To make the code easier to understand, the exception raising could replace the loop exit inside the following "if" block in SequenceUpdater.getCurrentValueAndAdvance():

if (
(_lockTimeoutInMillis >= 0L) &&
( (System.currentTimeMillis() - startTime.longValue()) > _lockTimeoutInMillis )
)

This approach was recommended by the discussion on DERBY-5423.