Index api consistency

[SpacemanPaul]

Reason for this pull request

1. API inconsistencies were identified during work on Abstract Base Classes for index backend interface(s). #1226.
2. redis-server was left out of Docker build in Migrate docker image off geobase to osgeo/gdal #1233, bypassing celery tests.

Proposed changes

1. Regularise API inconsistencies
2. Add redis-server to docker image to allow celery tests to run

Note: The celery integration depends on ancient and security-compromised version of celery. The general consensus seems to be that it is useless and nobody uses it, but I need to consult more widely before removing it all together.

[codecov]

Codecov Report

Merging #1234 (b9681a6) into develop (c1f20f7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1234   +/-   ##
========================================
  Coverage    92.71%   92.71%           
========================================
  Files          103      103           
  Lines        10595    10598    +3     
========================================
+ Hits          9823     9826    +3     
  Misses         772      772           

Impacted Files	Coverage Δ
datacube/index/abstract.py	94.89% <100.00%> (ø)
datacube/index/postgres/_datasets.py	94.84% <100.00%> (+0.02%)	⬆️
datacube/scripts/dataset.py	88.23% <100.00%> (+0.03%)	⬆️

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[omad]

LGTM