fix: dispatch OnStateChange outside mutex to prevent deadlock

[nghiack7]

Problem

Fixes #37

OnStateChange was called from inside setState while cb.mutex was held. Any CircuitBreaker method invoked from within the callback — Counts(), State(), Execute() — tries to acquire the same mutex and dead-locks.

// Before — deadlock if callback calls cb.Counts()
OnStateChange: func(name string, from, to gobreaker.State) {
    counts := cb.Counts()  // ← acquires mutex → deadlock
    log.Printf("state changed: %v counts: %v", to, counts)
},

Root cause

setState is always called while cb.mutex is held (via beforeRequest, afterRequest, or State). It called cb.onStateChange synchronously under that lock.

Fix

Record the pending notification in a new pendingNotify field (set under the lock in setState), then dispatch it after unlocking in the three public entry points that can trigger a state change (State, beforeRequest, afterRequest).

The notification is captured and cleared while the lock is still held, so concurrent goroutines cannot steal each other's notifications. Dispatching outside the lock allows the callback to freely call back into the breaker.

// After — safe to call any CircuitBreaker method in the callback
OnStateChange: func(name string, from, to gobreaker.State) {
    counts := cb.Counts()  // ✓ no deadlock
    log.Printf("state: %s → %s  counts: %v", from, to, counts)
},

Tests

Added TestOnStateChangeNoDeadlock: creates a breaker whose OnStateChange callback calls cb.Counts() and cb.State(), triggers a closed→open transition, then waits for the test to complete within 2 seconds. If the deadlock were present the test would time out.