Production block comparison facility

The block comparison facility presented earlier by Heikki would not be
able to be used in production systems. ISTM that it would be desirable
to have something that could be used in that way.

ISTM easy to make these changes

* optionally generate a FPW for every WAL record, not just first
change after checkpoint
full_page_writes = 'always'

* when an FPW arrives, optionally run a check to see if it compares
correctly against the page already there, when running streaming
replication without a recovery target. We could skip reporting any
problems until the database is consistent
full_page_write_check = on

The above changes seem easy to implement.

With FPW compression, this would be a usable feature in production.

Comments?

--
Simon Riggs http://www.2ndQuadrant.com/
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