[Core] Externalize a shared Mempool interface & implementation

[Olshansk]

Objective

Make the Mempool interface and implementation a common high-level type that can be reused across different modules

Origin Document

This discussion in #374: #374 (comment)

Goals

• Create a Mempool type that other modules can easily import & reuse
• Clean up the code so all the modules are using the same mempool implementation

Deliverable

• Create a shared mempool object based on utility/types/Mempool into shared/mempool
• Import and use shared/mempool in the utility module
• Import and use shared/mempool in the p2p module
• Add a couple mempool specific tests
• In utility/test/module_test.go: TODO(#388): Expose a GetMempoolfunction inutility_module so we can remove this reflection.

Non-goals / Non-deliverables

• Modify existing business logic

General issue deliverables

• Update the appropriate CHANGELOG(s)
• Update any relevant local/global README(s)
• Update relevant source code tree explanations
• Add or update any relevant or supporting mermaid diagrams

Testing Methodology

• **Mempool tests **: make test_mempool
• All tests: make test_all
• LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md

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Creator: @Olshansk
Co-Owners: @deblasis