Fix management of fn_extra caching during repeated GiST index scans.

Commit d22a09dc introduced official support
for GiST consistentFns that want to cache data using the FmgrInfo fn_extra
pointer: the idea was to preserve the cached values across gistrescan(),
whereas formerly they'd been leaked.  However, there was an oversight in
that, namely that multiple scan keys might reference the same column's
consistentFn; the code would result in propagating the same cache value
into multiple scan keys, resulting in crashes or wrong answers.  Use a
separate array instead to ensure that each scan key keeps its own state.

Per bug #8143 from Joel Roller.  Back-patch to 9.2 where the bug was
introduced.