perf(core-database-postgres): drop redundant and add missing index on blocks

[vasild]

The "blocks_unique" on (height, generator_public_key) is redundant
because we already have an index on (height) alone.

Also, we do queries like:

SELECT count(*) FROM blocks WHERE generator_public_key = '...'; [1]

and thus we need an index on (generator_public_key), add that missing
one.

The removal of the redundant index does not change timings:
selects that return small count: ~500ms
selects that return big count: ~500ms

The addition of an index on (generator_public_key) changes it to:
selects that return small count: ~50ms
selects that return big count: ~500ms

Proposed changes

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #1634 into develop will decrease coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1634      +/-   ##
===========================================
- Coverage    74.87%   74.74%   -0.13%     
===========================================
  Files          439      439              
  Lines         7896     7896              
  Branches      1039     1039              
===========================================
- Hits          5912     5902      -10     
- Misses        1764     1771       +7     
- Partials       220      223       +3

Impacted Files	Coverage Δ
...ges/core-database-postgres/lib/migrations/index.js	100% <ø> (ø)	⬆️
packages/core-forger/lib/manager.js	71.84% <0%> (-8.74%)	⬇️
...p2p/lib/server/versions/internal/handlers/utils.js	80% <0%> (-6.67%)	⬇️

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[codecov-io]

Codecov Report

Merging #1634 into develop will decrease coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1634      +/-   ##
===========================================
- Coverage    74.87%   74.74%   -0.13%     
===========================================
  Files          439      439              
  Lines         7896     7896              
  Branches      1039     1039              
===========================================
- Hits          5912     5902      -10     
- Misses        1764     1771       +7     
- Partials       220      223       +3

Impacted Files	Coverage Δ
...ges/core-database-postgres/lib/migrations/index.js	100% <ø> (ø)	⬆️
packages/core-forger/lib/manager.js	71.84% <0%> (-8.74%)	⬇️
...p2p/lib/server/versions/internal/handlers/utils.js	80% <0%> (-6.67%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8d1fc28...4d7ea52. Read the comment docs.