Re-work prepared geometry to use a custom memory context

[pramsey]

By using a custom memory context underneath the function manager context, we can free the GEOS objects cleanly at the end of the statement. Right now the GEOS objects are directly inside the fmgr context, so there's no way to clean them up before the context is freed. See the transform() code for an example.

Until this is done, the preparedgeometry will leak two objects for every SQL call, no matter how clean GEOS gets.

[pramsey]

Alternate approach using a child MemoryContext of the fmgr context to trigger a callback and clean-up, following example in transform cache.