refactor(core-database-postgres): add type hints and return types

[faustbrian]

Proposed changes

Add interfaces, types, type hints and return types to core-database-postgres.

Types of changes

• Refactoring (improve a current implementation without adding a new feature or fixing a bug)

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes

[ghost]

The ci/circleci: test-node10-unit job is failing as of d808fd72ce62499e783f52285bbe92db8e975801. Please review the logs for more information.

Once you've pushed the fixes, the build will automatically re-run. Thanks!

[codecov-io]

Codecov Report

Merging #2421 into 2.4 will increase coverage by 0.03%.
The diff coverage is 63.01%.

@@            Coverage Diff             @@
##              2.4    #2421      +/-   ##
==========================================
+ Coverage   66.31%   66.34%   +0.03%     
==========================================
  Files         398      398              
  Lines        8565     8540      -25     
  Branches      392      436      +44     
==========================================
- Hits         5680     5666      -14     
+ Misses       2839     2827      -12     
- Partials       46       47       +1

Impacted Files	Coverage Δ
packages/core-api/src/repositories/blocks.ts	51.85% <ø> (+1.85%)	⬆️
packages/core-api/src/repositories/repository.ts	90% <ø> (ø)	⬆️
packages/core-api/src/repositories/transactions.ts	83.33% <0%> (+1.88%)	⬆️
packages/core-database/src/database-service.ts	27.48% <0%> (ø)	⬆️
.../core-database-postgres/src/repositories/rounds.ts	71.42% <0%> (-3.58%)	⬇️
...ase/src/repositories/blocks-business-repository.ts	80% <100%> (-1.25%)	⬇️
...e-database-postgres/src/repositories/migrations.ts	100% <100%> (ø)	⬆️
...ges/core-database-postgres/src/models/migration.ts	100% <100%> (ø)	⬆️
...c/repositories/transactions-business-repository.ts	97.01% <100%> (-0.09%)	⬇️
.../core-database-postgres/src/postgres-connection.ts	57.94% <100%> (ø)	⬆️
... and 10 more

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