patch: update README-SSI

The attached patch updates README-SSI. In addition to some minor edits,
changes include:

 - add a section at the beginning that more clearly describes the SSI
   rule and defines "dangerous structure" with a diagram. It describes
   the optimizations we use about the relative commit times, and the
   case where one transaction is read-only. It includes a proof for the
   latter (novel) optimization, per Heikki's request.

 - note that heap page locks do not lock "gaps" like index pages

 - be clear about what's been implemented (parts of the README used the
   future tense, probably because they were written long ago), and
   remove a couple items from the "R&D Issues" list that have since
   been addressed.
   
Dan

-- 
Dan R. K. Ports              MIT CSAIL                http://drkp.net/