Speed up ring building in ValidateTopology

[strk]

The most expensive step in topology validation at the moment is building all rings.

In a topology with 20557 rings the current code on my machine takes 114 seconds to compute them all.

Current code makes 20577 iterations of a loop, and each iteration performs an INSERT and a DELETE statement, among other things.

[strk]

Additional operation in each iteration is the actual build of ring.

Temptative improvement (computing all rings in a single statement):

​https://gitlab.com/postgis/postgis/-/merge_requests/45

[strk]

New attempt looks promising: ​https://gitlab.com/postgis/postgis/-/merge_requests/46 Builds all rings in 25 seconds rather than 114 seconds

[Sandro Santilli <strk@…>]

In 406ecaa/git:

ValidateTopology: build all rings in a single statement

Closes #4952

Implements GetRingEdges internally, to avoid the subtransaction
while still surviving non-closed rings.

In real world case it reduces the cost of building 20356 rings
from ~100 seconds to ~20 seconds