Alembic-managed schema migrations for postgis driver. (cleaned up git history)

[SpacemanPaul]

Reason for this pull request

The database schema for the postgres database has been frozen for years because making schema changes requires manually writing code to check whether an update is required and applying said update.

Alembic is a schema migration framework for SQLAlchemy that automates much of this.

Proposed changes

• datacube system init for the postgis index driver now uses Alembic to detect what schema upgrades are required and apply them.
• Developer documentation on how to handle schema upgrades in future.

N.B. Some remaining coverage gaps that can't really be filled until we have some actual migrations lined up.

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

[codecov[bot]]

Codecov Report

Attention: 47 lines in your changes are missing coverage. Please review.

Comparison is base (5be0e6e) 85.37% compared to head (bd72270) 85.16%.

Files	Patch %	Lines
datacube/drivers/postgis/alembic/env.py	42.18%	37 Missing :warning:
datacube/drivers/postgis/_core.py	70.37%	8 Missing :warning:
.../alembic/versions/48fb71ece268_initial_revision.py	80.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1520 +/- ## =============================================== - Coverage 85.37% 85.16% -0.22% =============================================== Files 137 139 +2 Lines 14784 14895 +111 =============================================== + Hits 12622 12685 +63 - Misses 2162 2210 +48 ```

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