Improve ST_DWithin performance, especially for geography

[realityexists]

ST_DWithin is a very handy function (especially for geography, since it's one of the few functions that support geography "properly"), but it's often too slow to be useful. Please make it faster.

It's currently using a brute-force approach, but Paul mentioned an O(N+M) algorithm:  http://postgis.refractions.net/pipermail/postgis-users/2012-January/032012.html

[pramsey]

This is substantially complete for geography at r10006. The same approach could speed up distance searches in geometry. There is also a little more work to do adding some regression tests and potentially expanding the use of the tree cache to the ST_Covers(geog, geog) function.

[pramsey]

At r10044 there are now regression tests and the caching behavior has become the default.