Postgis driver cleanup and bug fix

[SpacemanPaul]

Reason for this pull request

• Postgis driver is still reporting itself as "experimental" and "unstable"
• Count methods in postgis driver did not support geospatial search.

Proposed changes

• Remove warning from postgis driver.
• Add geospatial search to count methods in postgis driver

• cherrypick COG fixes from 1.8 - will probably replace with odc-geo implementation before final 1.9.0 release.

  • [x] Tests added / passed
  • [x] Fully documented, including docs/about/whats_new.rst for all changes

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.16%. Comparing base (3d356c6) to head (fd7cf6e).

Files	Patch %	Lines
datacube/drivers/postgis/_api.py	76.19%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1608 +/- ## ============================================ Coverage 85.15% 85.16% ============================================ Files 141 141 Lines 15564 15574 +10 ============================================ + Hits 13254 13264 +10 Misses 2310 2310 ```

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