pgsql: Update README-SSI. Add a section to describe the "dangerous stru

Update README-SSI. Add a section to describe the "dangerous structure" that
SSI is based on, as well as the optimizations about relative commit times
and read-only transactions. Plus a bunch of other misc fixes and
improvements.

Dan Ports

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/78475b0ecac4e7793366d2dba0cb875a8fde6bb0

Modified Files
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src/backend/storage/lmgr/README-SSI |  204 ++++++++++++++++++++---------------
1 files changed, 117 insertions(+), 87 deletions(-)