Remove common_timestamp function from postgis driver

[SpacemanPaul]

Reason for this pull request

The common_timestamp custom postgresql function was previously used in both postgis and postgres index drivers to ensure correct timezone handling when inserting and extracting time data. The use of this function significantly complicated efficient indexing of time fields in both drivers.

With SQLAlchemy2, most of the functionality of the common_timestamp function can be handled with a simple type cast. Some complexities regarding handling of:

a) data with no explicit timezone marker,
b) databases with a default timezone other than UTC, and
c) the "find duplicates with time" method

necessitated some further internal restructure and changes to internal APIs.

Due to these complexities, I have only applied these changes to the postgis driver.

• Closes Get rid of common_timestamp function in 1.9.x #1584
• Tests added / passed
• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.24%. Comparing base (eeb5e6d) to head (06b3606).

Files with missing lines	Patch %	Lines
datacube/drivers/postgis/_api.py	92.10%	3 Missing ⚠️
datacube/drivers/postgis/_fields.py	93.47%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1623      +/-   ##
===============================================
+ Coverage        85.23%   85.24%   +0.01%     
===============================================
  Files              144      144              
  Lines            15633    15685      +52     
===============================================
+ Hits             13325    13371      +46     
- Misses            2308     2314       +6     

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