ST_EstimatedExtent bound to index

[pramsey]

Where possible, read the head of the index to get an estimate of the spatial extent.

[robe]

if you can get this done by next week push back. if not — its gonna have to wait.

[pramsey]

In 16149:

Proof of content for reading table extent from
the top of the index. Can be called with
_postgis_index_extent(tbl regclass, col text)
and returns box2d.

Currently not added to the st_estimatedextent()
code, because it has issues with indexes that
have EMPTY added to them. This is a symptom
of #2487. While it seems impossible to every
allow index-assisted searches for EMPTY
geometry, it will be necessary to create
invalid keys for them (NaN, NaN, NaN, Nan)
and then handle those keys so they don't
pollute the index with strange bounds.

References #2256

[pramsey]

Updated empty keys to use NaN boxes in r16151, and added index read to ST_EstimatedExtent() code.