pg_statistic_ext.staenabled might not be the best column name

I'd been thinking that staenabled is not the most suitable column name
for storing the types of statistics that are defined for the extended
statistics. For me, this indicates that something can be disabled,
but there's no syntax for that, and even if there was, if we were to
enable/disable the kinds, we'd likely want to keep tabs on which kinds
were originally defined, otherwise it's more of an ADD and DROP than
an ENABLE/DISABLE.

"stakind" seems like a better name. I'd have personally gone with
"statype" but pg_statistic already thinks stakind is better.

A patch which changes this is attached

--
David Rowley http://www.2ndQuadrant.com/
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