Close() previously closed the done channel and returned immediately,
without waiting for worker goroutines to exit. This caused goroutine
leaks in tests and allowed workers to race with connection teardown.
The ticker was also never stopped, leaking its internal goroutine.
Add a sync.WaitGroup to track the doWork goroutine and every worker
it spawns. Close() now calls wg.Wait() after signalling shutdown,
ensuring all goroutines have exited before tearing down connections.
Also stop the ticker to prevent resource leaks.
Document that a Batcher must not be reused after Close().
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