fix: relayer deadlock prevention


**Description**
Async locking.

**Problem Statement**
System hang risk.

**Technical Requirements**
- [ ] Mutex/Async guard.

**Implementation Guide**
- **Suggested Approach:** Non-blocking locks.
- **Affected Areas:** src/

**Acceptance Criteria**
- [ ] No deadlocks in stress.

**Security & Audit Considerations**
- [ ] None.

**Definition of Done**
- [ ] Stress test pass.