Replace Mutex<HashMap> with scc::HashMap in chainlink module

[thlorenz]

Context

Originated from this comment by @bmuddha
on PR #931.

Description

The chainlink module currently uses Mutex<HashMap> in several places for managing subscriptions and related state.
These could be replaced with scc::HashMap to move toward a lock-free design, improving performance and reducing
contention in the system.

This is identified as a low-hanging fruit refactoring task that can be tracked separately and completed during
the upcoming refactoring phase.

Tasks

• Identify all instances of Mutex<HashMap> in the chainlink module
• Evaluate scc::HashMap as a replacement
• Replace Mutex<HashMap> with scc::HashMap where applicable
• Test and verify performance improvements