refactor(core-transaction-pool): add type, interfaces and sort out structural issues

[faustbrian]

Proposed changes

Adds more types and interfaces and also sorts out some structural issues.

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 b319913ce5593179f4d4bec2adc33e3e8d6e48cb. 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 #2448 into 2.4 will decrease coverage by 0.34%.
The diff coverage is 92.23%.

@@            Coverage Diff            @@
##              2.4   #2448      +/-   ##
=========================================
- Coverage   68.25%   67.9%   -0.35%     
=========================================
  Files         387     386       -1     
  Lines        8445    8403      -42     
  Branches      393     435      +42     
=========================================
- Hits         5764    5706      -58     
- Misses       2631    2644      +13     
- Partials       50      53       +3

Impacted Files	Coverage Δ
packages/core-api/src/repositories/transactions.ts	83.33% <ø> (ø)	⬆️
...core-transactions/src/handlers/second-signature.ts	90% <0%> (ø)	⬆️
packages/core-transactions/src/utils.ts	75% <0%> (+15%)	⬆️
...transactions/src/handlers/delegate-registration.ts	44.82% <0%> (ø)	⬆️
packages/core-transactions/src/handlers/vote.ts	82.6% <0%> (ø)	⬆️
...ackages/core-transactions/src/handlers/transfer.ts	66.66% <0%> (ø)	⬆️
packages/core-database/src/database-service.ts	26.57% <0%> (ø)	⬆️
...ages/core-transactions/src/handlers/transaction.ts	75.86% <0%> (ø)	⬆️
...ain/src/processor/handlers/accept-block-handler.ts	86.36% <100%> (ø)	⬆️
...c/repositories/transactions-business-repository.ts	97.01% <100%> (ø)	⬆️
... and 17 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov-io]

Codecov Report

Merging #2448 into 2.4 will decrease coverage by 0.34%.
The diff coverage is 92.23%.

@@            Coverage Diff            @@
##              2.4   #2448      +/-   ##
=========================================
- Coverage   68.25%   67.9%   -0.35%     
=========================================
  Files         387     386       -1     
  Lines        8445    8403      -42     
  Branches      393     435      +42     
=========================================
- Hits         5764    5706      -58     
- Misses       2631    2644      +13     
- Partials       50      53       +3

Impacted Files	Coverage Δ
packages/core-api/src/repositories/transactions.ts	83.33% <ø> (ø)	⬆️
...core-transactions/src/handlers/second-signature.ts	90% <0%> (ø)	⬆️
packages/core-transactions/src/utils.ts	75% <0%> (+15%)	⬆️
...transactions/src/handlers/delegate-registration.ts	44.82% <0%> (ø)	⬆️
packages/core-transactions/src/handlers/vote.ts	82.6% <0%> (ø)	⬆️
...ackages/core-transactions/src/handlers/transfer.ts	66.66% <0%> (ø)	⬆️
packages/core-database/src/database-service.ts	26.57% <0%> (ø)	⬆️
...ages/core-transactions/src/handlers/transaction.ts	75.86% <0%> (ø)	⬆️
...ain/src/processor/handlers/accept-block-handler.ts	86.36% <100%> (ø)	⬆️
...c/repositories/transactions-business-repository.ts	97.01% <100%> (ø)	⬆️
... and 17 more

---

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 ebc99ba...6812b8d. Read the comment docs.