SSI tuning points

The attached patch addresses one of the open non-blockers for beta3.

These are tuning points which emerged in testing. The first is more
likely to be helpful. The second may be very important in a few
types of transaction mixes, but I threw in a lot of weasel words and
qualifiers because someone could easily try this to bring down the
transaction retry rate, but suffer a net loss in throughput because
less efficient plans could be chosen. I hope I made that point in a
reasonable fashion, although I'm certainly open to suggestions for
better wording.

-Kevin