perf(core-database-postgres): add index on transactions (timestamp)

[vasild]

Impact on queries:

SELECT block_id, serialized FROM transactions ORDER BY timestamp DESC OFFSET 0 LIMIT 25;
~1000ms -> ~100ms

SELECT
type,
MIN(fee) AS minFee,
MAX(fee) AS maxFee,
AVG(fee) AS avgFee,
MAX(timestamp) AS timestamp
FROM transactions
WHERE timestamp >= 53823000
GROUP BY type
ORDER BY timestamp DESC;
~350ms -> ~50ms

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 #1636 into develop will decrease coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1636      +/-   ##
===========================================
- 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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